Day of the Week Calculator

Give this calculator any date and it returns the weekday it falls on, where that date sits in the year, how many of that weekday the year holds, and the month drawn out around it.

For August 3, 2025 it reports a Sunday: the 215th day of 365, with 150 left, and the 31st Sunday out of 52, with 21 still to come. The month underneath marks the date in its calendar so the surrounding week is visible at a glance.

ISO 8601

The International Organization for Standardization maintains ISO 8601, the international standard covering the exchange of date and time data. Most of the world uses it, particularly for anything machine-readable, though adherence is not universal and plenty of everyday writing ignores it.

Two of its decisions matter here. Monday is the first day of the week under ISO 8601, which is not what the calendars in this article show, since the grid above starts on Sunday in the American convention. And weeks are numbered, with week 1 being the week containing the first Thursday of the year.

That numbering rule produces results that look wrong until you know it. January 1, 2000 belongs to week 52 of 1999, and December 31, 2025 belongs to week 1 of 2026. A date near New Year can legitimately carry a week number from the neighbouring year, which is why the calculator prints the ISO year alongside the week.

Working it out without a calculator

The standard algorithm for the weekday of any date is Zeller's congruence, published by Christian Zeller in the 1880s. It takes the day, month and year, shifts January and February to the end of the previous year so leap days fall last, and reduces the whole thing modulo 7. It is exact and it is awkward to do in your head.

For mental arithmetic the doomsday rule is easier. Each year has an anchor weekday, and a set of dates always land on it: 4/4, 6/6, 8/8, 10/10, 12/12, plus 5/9, 9/5, 7/11 and 11/7, and the last day of February. In 2025 that anchor is Friday, so April 4 and June 6 are both Fridays, and any nearby date can be counted off in sevens from there.

Both rest on the same fact: 365 days is 52 weeks and one day. A date therefore moves forward one weekday each year, and two across a leap year, which is why your birthday drifts through the week rather than staying put.

The Gregorian calendar

The calendar in general use is the Gregorian, named after Pope Gregory XIII, who introduced it in 1582. It is a solar calendar: the year tracks the Earth's orbit around the Sun rather than the phases of the Moon. Months are a leftover of older lunar reckoning, which is why twelve of them do not divide the year evenly and why their lengths are so uneven.

It replaced the Julian calendar, which treated the year as exactly 365.25 days and so ran slow by about 11 minutes a year. Over 1,600 years that drift had pushed the spring equinox ten days off. The Gregorian fix was to drop three leap days every four centuries: a year is a leap year if it divides by 4, unless it is a century year that does not divide by 400. 1600 and 2000 were leap years; 1700, 1800, 1900 and 2100 are not.

That gives an average year of 365.2425 days, close enough to the solar year to stay accurate for thousands of years. It also makes the calendar repeat exactly every 400 years: that span is 146,097 days, which is precisely 20,871 weeks. Any date in 2025 falls on the same weekday as the same date in 2425.

Where the names come from

The days are named for the seven classical planets, a scheme that spread through the Roman world during the Hellenistic period, between the death of Alexander the Great in 323 BC and the rise of the Roman Empire in 31 BC. In Latin, Sunday belonged to Sol, Monday to Luna, Tuesday to Mars, Wednesday to Mercurius, Thursday to Jove, Friday to Venus and Saturday to Saturnus.

The Latin names carried into the Romance languages more or less intact, which is why French has mardi, mercredi, jeudi and vendredi. English took a detour. Sunday, Monday and Saturday kept their bodies, but the four midweek days were given the nearest Germanic gods instead: Tiw for Mars, Woden for Mercury, Thor for Jupiter and Frigg for Venus. The week is the same planetary week; only the gods were swapped.

English	Latin, and the body it names	Named in English after
Sunday	dies Solis, the Sun	Sun
Monday	dies Lunae, the Moon	Moon
Tuesday	dies Martis, Mars	Tiw
Wednesday	dies Mercurii, Mercury	Woden
Thursday	dies Iovis, Jupiter	Thor
Friday	dies Veneris, Venus	Frigg
Saturday	dies Saturni, Saturn	Saturn

The scheme travelled further than Europe. Japanese uses the classical Chinese five elements mapped onto the same planets, so Friday is 金曜日, from kinsei meaning Venus and yōbi meaning day of the week, and Tuesday is 火曜日, literally fire day, fire being the element of Mars. In much of India, Thursday is Guruvāra, after Bṛhaspati, preceptor of the gods and regent of Jupiter.

A note on each day

Monday is the ISO first day of the week and, unusually, an anagram of a single ordinary word: dynamo. In finance the long-observed Monday effect describes Monday returns as historically the weakest of the week rather than the strongest, an anomaly that has weakened as markets have become more efficient.

Tuesday carries Black Tuesday, October 29, 1929, the heaviest day of the Wall Street crash. It came after Black Thursday and Black Monday of the same week, and the three together opened the Great Depression.

Wednesday is Hump Day in American usage, the midpoint of the working week. Ash Wednesday is the start of Lent, and in Australia it also names the bushfires of February 16, 1983, which killed 75 people across Victoria and South Australia and remain among the country's worst disasters.

Thursday holds Maundy Thursday, the Thursday before Easter, commemorating the Last Supper. Thanksgiving in the United States is fixed to the fourth Thursday of November, which is what makes the following Friday so commercially significant.

Friday gathers the most superstition. Friday the 13th is widely held to be unlucky in English-speaking countries, while Italy assigns that role to Friday the 17th and treats 13 as fortunate. Good Friday marks the crucifixion. Black Friday, the day after Thanksgiving, opens the Christmas shopping season, and it turned genuinely dangerous in 2008 when a worker was killed in a crowd surge at a New York store opening.

Saturday is Shabbat in Israel, when most businesses and public transport stop. Australia and New Zealand hold general elections on Saturdays. Sweden has lördagsgodis, the Saturday sweets convention that grew out of a mid-century dental health campaign and left most Swedish children eating confectionery on one day a week.

Sunday is the first day of the week in the United States, Israel and much of the Middle East, and the last under ISO 8601. It is the Christian day of rest and worship, banks close almost everywhere, and the NFL championship gives American English the phrase Super Bowl Sunday.

Patterns worth knowing

Every month that begins on a Sunday contains a Friday the 13th, because the 13th is always twelve days after the 1st and twelve is five past a full week. Over the full 400-year Gregorian cycle the 13th lands on a Friday 688 times, slightly more often than on any other weekday, with Thursday and Saturday tied at the bottom on 684 each. The margin is small but it is real, and it is a consequence of the leap-year rule rather than of anything sinister.

A common year holds 52 of each weekday plus one extra, and that extra is whichever day January 1 falls on. A leap year has two extras. It is why 2025, starting on a Wednesday, has 53 Wednesdays and 52 of everything else.

The same arithmetic explains why a year never starts and ends on different weekdays in a common year: January 1 and December 31 of 2025 are both Wednesdays.