Day of the Week Calculator

About the Day of the Week Calculator

Enter a day, month and year to see which day of the week it falls on — the day you were born, a historical event or a date far in the future. The answer comes with the working of John Conway’s Doomsday rule, so you can check it, or learn to do it in your head.

The same page checks whether a year is a leap year (with the reason), lists the years in which a date falls on a chosen weekday — useful for finding when a birthday next lands on a Saturday — and shows every Friday the 13th of a year. Dates before the calendar reform can be read in the Julian calendar, with the switch in 1582 or the British switch in 1752.

Common uses

• Finding the day of the week you or someone else was born on.
• Checking the weekday of a historical date, a deadline or a future event.
• Planning when a birthday or anniversary falls on a weekend in coming years.
• Learning and practising the Doomsday rule for mental calculation.
• Converting an Old Style (Julian) date from old records to the modern calendar.

How the Doomsday rule works

In every year, a few easy-to-remember dates fall on the same weekday, called that year’s doomsday: 4/4, 6/6, 8/8, 10/10 and 12/12, 9/5 and 5/9, 11/7 and 7/11, 14 March, the last day of February, and 3 January (4 January in a leap year). In 2026 they are all Saturdays.

• Century anchor: for the Gregorian calendar, (5 × (century mod 4) + 2) mod 7, counting Sunday = 0. The 1900s start from Wednesday and the 2000s from Tuesday.
• Year: split the last two digits into dozens, the remainder, and the number of fours in the remainder. For 1947: 47 = 3 dozens + 11, and 11 holds 2 fours. Anchor + 3 + 11 + 2 = 3 + 16 = 19, and 19 mod 7 = 5, so Friday is the doomsday of 1947.
• Date: count from the nearest doomsday of the month. 15 August is 7 days after 8 August, so it is also a Friday.

In the Julian calendar the century anchor is 6 × century mod 7. The calculator checks every answer against the Julian Day Number, the continuous count of days astronomers use. The method is John H. Conway’s, published as “Tomorrow is the day after doomsday” in Eureka 36 (1973).

Julian and Gregorian dates

The Julian calendar adds a leap day every fourth year, which made it drift about three days every 400 years. The papal bull Inter gravissimas (1582) introduced the Gregorian calendar, which skips the leap day in century years not divisible by 400, and dropped ten days: Thursday 4 October 1582 was followed by Friday 15 October 1582. Great Britain and its colonies changed under the Calendar (New Style) Act 1750, when Wednesday 2 September 1752 was followed by Thursday 14 September 1752; other countries changed on other dates (Russia, for example, in 1918).

The weekday cycle was never interrupted. From 14 March 1900 until 13 March 2100 (Gregorian dates) the Julian calendar is 13 days behind the Gregorian one: Saturday 3 October 2026 is Saturday 20 September 2026 in the Julian calendar.

Leap years

In the Gregorian calendar a year is a leap year if it is divisible by 4 — except century years, which are leap years only when divisible by 400. So 2024 and 2000 are leap years, while 1900 and 2100 are not. In the Julian calendar every year divisible by 4 is a leap year, including 1700, 1800 and 1900.

Limitations

• Years run from 1 to 9999; years before AD 1 (BC dates) are not supported.
• Each year starts on 1 January. In England before 1752 the year number changed on 25 March, so old records may write 11 February 1731 for a date that is entered here as 11 February 1732 (Julian).
• Only the 1582 and 1752 switches are built in. For a country that changed on another date, choose Gregorian or Julian according to the calendar the date was written in.

Privacy

Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.