Day of the Week Calculator - What Day Was It?

About the Day of the Week Calculator

What day of the week were you born? What day was a famous historical event? What day will a wedding date, a deadline or New Year's Day fall on years from now? The day of the week calculator answers instantly for any date in the Gregorian calendar, past or future.

As well as the weekday, it gives the day of the year, how many days are left in the year, and the weekdays of the same date in nearby years. For anyone curious about how it works, it shows the full working of Zeller's congruence, a classic formula that finds the weekday of any date with simple arithmetic.

How to Use the Day of the Week Calculator

Enter a date.

The day of the week appears, with the day of the year and the formula working.

Zeller's Congruence

  h = ( d + ⌊13(m + 1) ÷ 5⌋ + K + ⌊K ÷ 4⌋ + ⌊J ÷ 4⌋ + 5J ) mod 7

  d = day of the month
  m = month (March = 3 … December = 12; January = 13 and February = 14
      of the previous year)
  K = year of the century (year mod 100)
  J = century (year ÷ 100, rounded down)

  h:  0 Saturday, 1 Sunday, 2 Monday, 3 Tuesday, 4 Wednesday, 5 Thursday, 6 Friday

Treating January and February as the 13th and 14th months of the previous year puts the leap day at the end of the "year", which keeps the formula simple.

Step-by-Step Example

20 July 1969 — the day of the first Moon landing.

  d = 20, m = 7, K = 69, J = 19
  ⌊13 × 8 ÷ 5⌋ = ⌊20.8⌋ = 20
  ⌊69 ÷ 4⌋ = 17,  ⌊19 ÷ 4⌋ = 4,  5 × 19 = 95
  h = (20 + 20 + 69 + 17 + 4 + 95) mod 7 = 225 mod 7 = 1
  → Sunday

It was the 201st day of 1969. The same date was a Saturday in 1968 and a Monday in 1970.

1 January 2000. January counts as month 13 of 1999, so d = 1, m = 13, K = 99, J = 19, and the formula gives h = 0: a Saturday.

The Doomsday Rule

Mathematician John Conway devised a mental method called the Doomsday rule. In any year, certain easy-to-remember dates all fall on the same weekday, the year's "doomsday": 4/4, 6/6, 8/8, 10/10 and 12/12, as well as 5/9, 9/5, 7/11, 11/7 and the last day of February. In 2026 the doomsday is Saturday, so 4 April, 6 June and 12 December 2026 are all Saturdays. From the nearest doomsday, count forward or back to any date. With practice, Conway could name the weekday of any date in a couple of seconds.

How Weekdays Shift From Year to Year

A common year has 365 days, which is 52 weeks and 1 day, so each date falls one weekday later the following year. A leap year adds a second day for dates after 29 February. In the example, 20 July moved from Saturday in 1968 to Sunday in 1969 — only one day, because the leap day came in February 1968, before the start of the year-long gap, then to Monday in 1970 and Tuesday in 1971, before jumping to Thursday in 1972 after 29 February 1972.

Calendars Repeat

Because of this pattern, calendars repeat. A common year's calendar can be reused 6 or 11 years later, and the whole Gregorian calendar repeats exactly every 400 years — 146,097 days, which is exactly 20,871 weeks. So 29 September 2426 will be a Tuesday, just like 29 September 2026. Within a century, the pattern of weekdays repeats every 28 years, provided no century year that is not a leap year falls in between.

Famous Dates and Their Weekdays

Some well-known dates and the days they fell on: the Moon landing on 20 July 1969 was a Sunday; 1 January 2000 was a Saturday; 11 November 1918, the Armistice ending the First World War, was a Monday; and 14 August 1947, the date Pakistan marks as its founding, was a Thursday. Birthdays are the most popular dates to check — the old rhyme says Monday's child is fair of face and Tuesday's child is full of grace — and many people are surprised to learn which weekday they were born on.

Historical Dates

The calculator uses the Gregorian calendar, introduced in 1582 and adopted by different countries at different times — Britain and its colonies in 1752, Russia in 1918. Before that, many countries used the Julian calendar, which by the 18th century ran 11 days behind. Dates in historical records from before a country's switch may therefore give a different weekday unless they are first converted to the Gregorian calendar.

Understanding Your Result

The headline is the day of the week.

The date line gives the full date.

The day of the year line gives its position in the year and the days left.

The nearby years line gives the weekday of the same date in the previous year and the next three.

The formula line gives the result of Zeller's congruence.

When Should You Use This Calculator?

Use it to find the weekday you were born on.

Use it to check the weekday of historical events.

Use it to plan events and see which day of the week a future date falls on.

Use it to learn and check Zeller's congruence or the Doomsday rule.

Common Mistakes

Forgetting that January and February count as months 13 and 14 of the previous year. The formula gives the wrong day otherwise.

Mixing up the result numbering. In Zeller's form, 0 is Saturday.

Applying the Gregorian calendar to Julian dates. Convert historical dates first.

Assuming weekdays shift by one every year. Leap years add a second day.

Rounding the divisions the wrong way. Each ⌊ ⌋ in the formula means round down to the whole number below.