Day of the Week Calculator: What Day Was It?

Doomsday dates: the easy dates to remember in every month

Century anchor days

July 20 in the ten years after 1969

How to find the day of the week of a date in your head (Doomsday rule)

1. Find the century anchor. Compute (5 × (century mod 4) + 2) mod 7, with Sunday = 0. For the 1900s that is 3, a Wednesday; for the 2000s it is 2, a Tuesday.
2. Add the year part. Take the last two digits of the year. Divide by 12 (rounded down), add the remainder, then add the remainder divided by 4 (rounded down). For 1969: 5 + 9 + 2 = 16.
3. Get the doomsday of the year. Add the century anchor and the year part and take the result modulo 7. For 1969: 3 + 16 = 19, and mod 7 that is 5, a Friday.
4. Pick the month doomsday date. Use the table above. For July it is July 11, which is also a Friday.
5. Count forward to your date. The difference between your day and the doomsday date, modulo 7, is how many weekdays to move forward. 20 − 11 = 9, and that mod 7 is 2, so move 2 days forward from Friday: a Sunday.

Frequently asked questions

What day of the week was I born?

Enter your birth date. For example, July 20, 1969 was a Sunday. The calculator also shows which weekday occurrence in the month it was and, in the second box, the next years your birthday falls on a weekday you choose.

How does the Doomsday rule work?

John Conway devised it in the 1970s. In every year, a handful of easy dates (4/4, 6/6, 8/8, 10/10, 12/12, 5/9, 9/5, 7/11, 11/7, the last day of February and the 3rd or 4th of January and 14th of March) fall on the same weekday, called the doomsday. Work out the doomsday of the year from its century and last two digits, then count from the nearest doomsday date to your date. The calculator shows these steps for the date you enter.

How do I find which Saturday a birthday falls on?

Use the second box: pick the date and a weekday and it lists the next years that month and day falls on it. For July 20 after 1969, the next Saturday is in 1974. A date typically returns to the same weekday 5, 6 or 11 years later.

How is a date on February 29 handled?

February 29 exists only in leap years, so the next-years list and the weekday search count only leap years. A leap day repeats its weekday every 28 years most of the time, but century years such as 2100 are not leap years and break the cycle.

Is the weekday correct for dates before 1582?

It is correct for the proleptic Gregorian calendar, which extends the Gregorian rules backwards, as ISO 8601 and most software do. Historical records from before a country switched to the Gregorian calendar were dated in the Julian calendar, which runs days behind (10 days in 1582, 13 today). The calendar changed at different times: 1582 in Catholic Europe, 1752 in Britain and its colonies, 1918 in Russia and 1923 in Greece. For a historical date, convert it to Gregorian first.

Why does the weekday of a date move by one each year?

A common year has 365 days, which is 52 weeks and 1 day, so the same date falls one weekday later next year, or two when a leap day falls in between. The whole pattern repeats every 400 years, because 400 years contain exactly 146,097 days, a whole number of weeks.

What does "the 2nd Tuesday of the month" mean here?

The tool counts how many times that weekday has occurred in the month up to and including your date: days 1 to 7 are the first of their weekday, 8 to 14 the second, and so on. It also says whether your date is the last such weekday of the month, which matters for rules like "the last Monday in May".

Which ISO week is the date in?

ISO 8601 weeks start on Monday and week 1 holds the first Thursday of the year. The result shows the week and its code, such as 1969-W29. For US weeks and the dates a week number covers, see the week number calculator.