Concept

What is the day of the week?

What is the day of the week?

The day of the week is one of seven recurring named days — Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, and Sunday in English — that label the date within a continuous seven-day cycle. The cycle is independent of the month and the year: every Monday is followed by a Tuesday, regardless of whether the date crosses a month boundary or a year boundary. The same date in any future year falls on a determinable day of the week, computable from a fixed rule that requires only the date itself.2

The seven-day cycle is older than the Gregorian calendar, older than the Julian calendar, and older than the Christian church that propagated it through Europe. Almost every culture that uses civil calendars today uses the same seven-day cycle, and the days line up across them: a Wednesday on a Western civil calendar is a Wednesday on the Hebrew calendar, a Wednesday on the Islamic calendar, and a Wednesday on the Japanese calendar.2 The names of the days vary by language, but the cycle does not.

Where did the seven-day week come from?

The seven-day week has two distinct ancient ancestors that converged in the Roman Empire of the first centuries of the common era. The older is the Jewish week, which arranges six days of work followed by the Sabbath — a rest day fixed astronomically only by the cycle of seven, with no anchoring to the moon or any other celestial event. The younger is the planetary or astrological week of Hellenistic Mediterranean astrology, which assigned each hour of the day in turn to one of the seven moving celestial bodies known to the ancients (the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn) and named each day after the body that ruled its first hour.26

The two systems coexisted in the Roman Empire from at least the first century of the common era. The Jewish week supplied the discipline of a fixed weekly rest day; the planetary week supplied the names. By the early fourth century the seven-day cycle had displaced the older Roman eight-day market week (the nundinal cycle) in everyday life. The standard date marking its formal civil status is , when the emperor Constantine issued an edict that magistrates and townspeople were to rest "on the venerable day of the Sun" and that workshops in the cities were to close — the first imperial decree treating a day of the seven-day week as a public day of rest, preserved in the sixth-century Codex Justinianus.96

Constantine's edict did not invent the cycle, did not name it Christian (the day was named for the Sun, the chief deity of the late-Roman cult of Sol Invictus), and did not abolish work on the Sabbath. What it did do was lock the seven-day cycle into the operational calendar of the largest political entity in the western world. The cycle has run unbroken from then until now: the day of the week of any given date in the past 1,700 years can be calculated exactly from the rules of the calendar in force on that date, with no gaps or resets.2

What are the planetary names of the days?

The English names of the days of the week, alongside the Romance-language names that share their roots, are a direct inheritance from the planetary week of late antiquity. Each day was named for the celestial body that the Hellenistic astrologers placed at the head of its first hour, in an order derived from a particular rotation through the seven planets sorted by orbital period.26 Saturday was Saturn's day; Sunday was the Sun's; Monday was the Moon's. The remaining four were named for the planets the Romans called Mars, Mercury, Jupiter, and Venus — preserved in modern Romance languages as mardi / martedì / martes, mercredi / mercoledì / miércoles, jeudi / giovedì / jueves, and vendredi / venerdì / viernes.

The Germanic languages received the same week from the Romans but renamed the four planet-days after the Norse gods identified with the Roman ones: Tiw (the war god, for Mars) became Tuesday, Woden (for Mercury) became Wednesday, Thor (the thunder god, for Jupiter) became Thursday, and Freya (for Venus) became Friday. Saturday alone preserved the Roman planet's name in English — most other Germanic languages substituted the Norse name Laugardagr, "washing day", surviving in Norwegian and Swedish lördag, Danish lørdag, and German Sonnabend ("Sunday eve") in some regional usage.62

Does the week start on Monday or Sunday?

The week starts on different days in different places, and which day a calendar prints first is a regional convention rather than an astronomical or historical fact. The international standard for date strings — the standard that fixes the date format YYYY-MM-DD — also fixes the ISO week as starting on Monday, and numbers the days Monday = 1 through Sunday = 7. This is the convention used by business-day calendars, project planning software, and most of Europe.110

Locale data maintained by the Unicode Consortium — the same data set that drives every operating system's locale-aware calendar widget — records the actual first-day-of-week conventions in use across the world. Three groupings emerge from the data:5

  • Monday-first is the world default and the convention in most of Europe, Russia and the post-Soviet states, Australia and New Zealand, much of Latin America (including Argentina, Chile, Colombia, Ecuador, Peru, Uruguay), China, Vietnam, Malaysia, Turkey, and most of South Asia outside India. The locale data calls this mon and assigns it to the territory code 001 (the world).5
  • Sunday-first is the convention in the United States and Canada, most of Latin America's largest markets (Brazil, Mexico), Japan, South Korea, Taiwan, the Philippines, Hong Kong, Thailand, India, Israel, South Africa, and most of southern Africa. About sixty territories in the locale data use sun.5
  • Saturday-first is the convention in most of the Arab world: Bahrain, Djibouti, Egypt, Iraq, Iran, Jordan, Kuwait, Libya, Oman, Qatar, Saudi Arabia, Sudan, Syria, the United Arab Emirates, and Yemen. The Maldives is the one country whose calendar starts on Friday.5

None of these regional choices changes the underlying cycle. A Monday in Berlin is still a Monday in New York and a Monday in Cairo; the calendars merely arrange the same seven days into a column with a different first row. International standards for machine-readable dates resolve the ambiguity by fixing Monday as day 1 of the week regardless of locale.1 Locale-aware software exposes both: the underlying day-of-week index from the operating system's date library, and the locale's display preference from the locale data.5

How is the day of the week numbered?

Two conventions for numbering the days of the week are in everyday use, and they disagree on both the starting point and the starting number. The international date-format standard numbers the days Monday = 1, Tuesday = 2, Wednesday = 3, Thursday = 4, Friday = 5, Saturday = 6, Sunday = 7 — the form used in the ISO week date format YYYY-Www-D, in spreadsheet functions like WEEKDAY(date, 2), and in databases that follow the standard.1 The convention in the C standard library and its descendants is the opposite: Sunday = 0, Monday = 1, Tuesday = 2, Wednesday = 3, Thursday = 4, Friday = 5, Saturday = 6, with the cycle starting on Sunday and numbered from zero. This is the form returned by the tm_wday field of the C struct tm structure, by JavaScript's Date.prototype.getDay(), and by every operating system whose date library descends from Unix.34

The mismatch is a frequent source of off-by-one errors. A loop over the days of the week in JavaScript starts at index 0 (Sunday) and runs to 6 (Saturday); the same loop expressed against the international standard starts at 1 (Monday) and runs to 7 (Sunday). Code that takes a day index from one source and uses it as an offset against the other will be exactly two days out of phase. Newer date libraries — including JavaScript's Temporal proposal and Python's isoweekday() method on datetime — expose the international numbering directly to avoid the conversion step.1

Other numbering choices exist in narrower contexts. Hebrew uses an ordinal naming with Sunday = "first day"; Russian uses an ordinal naming with Monday = "second day" (the surviving artefact of an older ecclesiastical numbering that started the week on Sunday). Japanese, Korean, and Chinese names use the planetary day names as labels and number the days separately when needed. None of these affect the underlying seven-day cycle — they only affect what the days are called and what number their locale assigns.2

How do you compute the day of the week for any date?

The day of the week for any Gregorian date can be computed exactly from the date alone, with a single line of integer arithmetic. The classical algorithm is Zeller's congruence, published by the German mathematician Christian Zeller in 1882. Letting q be the day of the month, m the month (with January and February treated as months 13 and 14 of the previous year), and the year written as 100·J + K:7

h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ − 2J) mod 7

The result h is 0 for Saturday, 1 for Sunday, and so on through 6 for Friday. The strange-looking month shift is the trick that makes the formula work: by treating January and February as the last two months of the preceding year, the leap-day correction lands at the end of the year and the rest of the formula does not have to special-case it. The same arrangement appears in nearly every later day-of-the-week algorithm.710

An easier algorithm to perform mentally is the Doomsday rule, devised by the British mathematician John Horton Conway in 1973 and published with worked examples and mnemonics in the 1982 book Winning Ways for Your Mathematical Plays.8 The rule rests on the observation that several easy-to-remember dates fall on the same day of the week in any given year — 4 April, 6 June, 8 August, 10 October, 12 December (the "even doublets"), 9 May, 5 September, 11 July, 7 November ("I work from 9 to 5 at the 7-Eleven"), the last day of February, and 3 January in common years or 4 January in leap years. That weekday is called the year's Doomsday. To find the weekday of any date, identify the nearest Doomsday in the same month, count the offset to the target date, and add it to the Doomsday. For the years 2000–2099 the Doomsday of any year can be computed from the year alone in a few seconds of mental arithmetic, with the century anchor of Tuesday often remembered as "Y-Tue-K".8

Software almost never uses these algorithms directly. The dominant approach in computer science, popularised by the calendar-arithmetic textbook of Edward Reingold and Nachum Dershowitz, converts every date into a single integer day count from a fixed epoch — the R.D., "rata die" or "fixed date", with day 1 set to 1 January 1 of the proleptic Gregorian calendar — and then takes the day count modulo 7 to recover the weekday.10 Once a date library has the day count, every weekday question becomes a one-step modular reduction. Our own day-of-the-week calculator uses the same approach: convert the date to a day count, take the result mod 7, and look up the name.

How does the day-of-week cycle interact with the calendar?

The day of the week and the calendar date are linked by the rules of the Gregorian calendar — the civil calendar used internationally — which contains 97 leap years in every 400-year cycle. A common year of 365 days is 52 weeks and one day; the date of 1 January advances by one weekday in any such year. A leap year of 366 days is 52 weeks and two days; the date of 1 January advances by two weekdays. Across each 400-year cycle the calendar contains exactly 146,097 days, which is exactly 20,871 weeks. Because 146,097 is divisible by seven, the entire mapping of calendar dates to days of the week repeats every 400 years.12

The 400-year repeat is the calendar's deepest structural property and the reason every weekday-of-date question has a clean answer. is a Saturday; will be a Saturday; will be a Saturday. The same period also explains why some weekday-date pairs (Friday 13 January, for example, or 29 February falling on a Sunday) are noticeably more or less common over a 400-year span than naive 1-in-7 reasoning would predict — the cycle is not exactly evenly distributed across weekdays, and the small bias is reproducible from the rules of the calendar.1

The seven-day cycle interacts with two other date-counting systems worth knowing. The ISO week date rewrites the same date as a (year, week, weekday) triple — 2026-W19-6 for Saturday 9 May 2026 — using the ISO numbering of weekdays Monday = 1 through Sunday = 7. The two formats refer to identical days; a tool that consumes one can convert to the other without ambiguity, and our week-number lookup exposes both.1 The other system worth knowing is the calendar's relationship to weekday-recurring events like "the second Tuesday of every month" or "Memorial Day, the last Monday of May": the same 400-year cycle that fixes the weekday of any date also fixes the date of any "nth weekday of the month" rule.2

Frequently asked questions

What day of the week is it today?

The current day is shown by every operating system's clock and by our day-of-the-week calculator, which defaults to today's date and returns the full weekday name. The same calculator also resolves the weekday for any past or future date — the cycle is exactly determined by the date.10

Why are there seven days in the week?

For two converging historical reasons: the seven-day Jewish week, which fixed a six-day work cycle followed by a Sabbath, and the planetary week of Hellenistic astrology, which named one day after each of the seven moving celestial bodies known to ancient astronomers — the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. The two combined in the Roman Empire of the first three centuries of the common era and were locked into civil status by the emperor Constantine on .29

Has the seven-day cycle ever been interrupted?

Not in any calendar still in civil use. The Gregorian reform of 1582 deleted ten days from the calendar — was followed directly by in the Catholic states — but the day of the week ran straight through: the first date was a Thursday, and the second a Friday. Two attempts to abolish the seven-day cycle entirely — the French Revolutionary ten-day week of 1793–1805 and the Soviet five- and six-day weeks of 1929–1940 — were both abandoned and never spread beyond their issuing states.2

Why does Sunday come first in some calendars and last in others?

Because regional convention has not converged on one answer. Most of Europe, Australia, Russia, and much of Asia print Monday as the first day of the calendar; the United States, Canada, Japan, India, and most of the Arab world print Sunday or Saturday first. The Unicode Consortium's locale data records the first-day choice for every territory, and the international date-string standard picks Monday = 1 as the technical default.51

Why does my code think Sunday is day 0 but the standard says Monday is day 1?

Because the C standard library — which most date code in the world descends from — numbers the days Sunday = 0 through Saturday = 6, and the international date-format standard numbers them Monday = 1 through Sunday = 7. The two conventions disagree on both the starting day and the starting index. JavaScript's Date.prototype.getDay(), Python's weekday() (which uses Monday = 0 — a third convention), and the ISO week date are the three forms most commonly mixed up; converting between them by table is safer than by arithmetic.134

How can I find the day of the week for a date in my head?

Learn John Conway's Doomsday rule. The rule fixes a single weekday — the year's "Doomsday" — that several easy-to-remember dates (4 April, 6 June, 8 August, 10 October, 12 December, the last day of February) all fall on. From the Doomsday, every other date in that year is a small offset away. The full algorithm fits on an index card and resolves any 20th-or-21st-century date in a few seconds with practice.8

Footnotes

  1. 1. ISO 8601-1:2019, Date and time — Representations for information interchange — Part 1: Basic rules , International Organization for Standardization (2019) — accessed 2026-05-09.
  2. 2. The Seven Day Circle: The History and Meaning of the Week , Eviatar Zerubavel, University of Chicago Press (originally Free Press, 1985) (1989) — accessed 2026-05-09.
  3. 3. <time.h> — time types (struct tm), in The Open Group Base Specifications Issue 7, IEEE Std 1003.1-2017 , The Open Group / IEEE (2018) — accessed 2026-05-09.
  4. 4. ECMAScript 2024 Language Specification — 21.4.1.6 Week Day, and Date.prototype.getDay , Ecma International (Ecma-262, 15th edition) (2024) — accessed 2026-05-09.
  5. 5. Unicode Locale Data Markup Language (LDML) Part 4: Dates — Week Data, and supplementalData.xml , Unicode Consortium (Common Locale Data Repository) — accessed 2026-05-09.
  6. 6. Week , Encyclopædia Britannica — accessed 2026-05-09.
  7. 7. Die Grundaufgaben der Kalenderrechnung auf neue und vereinfachte Weise gelöst (Christian Zeller) , Württembergische Vierteljahrshefte für Landesgeschichte, vol. 5, pp. 313–314 (Stuttgart) (1882) — accessed 2026-05-09.
  8. 8. Winning Ways for Your Mathematical Plays, Volume 2: Games in Particular — Chapter 22, pp. 795–797 (the Doomsday rule) , Elwyn R. Berlekamp, John H. Conway, and Richard K. Guy, Academic Press, London (1982) — accessed 2026-05-09.
  9. 9. Codex Justinianus 3.12.2 — On the venerable day of the Sun (Constantine, 7 March 321) , Corpus Iuris Civilis, ed. Krueger (Berlin, 1877); English translation by S. P. Scott, The Civil Law (Cincinnati, 1932), vol. 12 (529) — accessed 2026-05-09.
  10. 10. Frequently Asked Questions about Calendars (sections on the week and day-of-week computation) , Claus Tøndering — accessed 2026-05-09.