What Day of the Week Was I Born? Find Out in Seconds
Find the weekday of any date, past or future, and learn the math behind it: Zeller's congruence with a worked example and the mental Doomsday rule.
Were you born on a Monday or a Saturday? On what day did the Moon landing happen? Will next Christmas fall on a weekend? Finding the day of the week for any date takes a second with a calculator, and the math behind it is a fun puzzle in its own right. Here is both.
The quick way
Enter the date in our day of the week calculator and it returns the weekday instantly, for any year. A few examples to try:
| Date | Day of the week |
|---|---|
| July 20, 1969 (Apollo 11 Moon landing) | Sunday |
| January 1, 2000 | Saturday |
| December 25, 2026 | Friday |
Why every year shifts by one or two days
A normal year has 365 days, which is 52 weeks plus 1 day. That is why your birthday moves forward by one weekday each year. A leap year has 366 days, or 52 weeks plus 2 days, so after a February 29 the shift is two days. That simple fact is the basis for every day-of-the-week method.
Zeller's congruence: the formula
In 1882, Christian Zeller published a formula that gives the weekday of any date in the Gregorian calendar:
h = (q + ⌊13(m + 1) / 5⌋ + K + ⌊K / 4⌋ + ⌊J / 4⌋ + 5J) mod 7- q: day of the month
- m: month, with March = 3 … December = 12, and January and February counted as months 13 and 14 of the previous year
- K: the year within the century (year mod 100)
- J: the century (year ÷ 100, rounded down)
- ⌊ ⌋: round down to a whole number
- h: the result, where 0 = Saturday, 1 = Sunday, 2 = Monday … 6 = Friday
Worked example: July 20, 1969
q = 20, m = 7, K = 69, J = 19.
- ⌊13 × (7 + 1) ÷ 5⌋ = ⌊104 ÷ 5⌋ = 20
- ⌊K ÷ 4⌋ = ⌊69 ÷ 4⌋ = 17
- ⌊J ÷ 4⌋ = ⌊19 ÷ 4⌋ = 4
- 5J = 95
- Sum: 20 + 20 + 69 + 17 + 4 + 95 = 225
- 225 mod 7 = 1 → Sunday
The Apollo 11 lunar module landed on Sunday, July 20, 1969, which matches.
The January/February adjustment is the step people forget. For February 14, 2026, use m = 14 and the year 2025 (K = 25, J = 20).
The Doomsday rule: do it in your head
Mathematician John Conway created a mental shortcut. Every year, a set of easy-to-remember dates all fall on the same weekday, called the "doomsday": 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 2026 the doomsday is Saturday. So April 4, June 6, August 8, October 10 and December 12, 2026 are all Saturdays. To find any other date, start from the nearest doomsday and count. December 25 is 13 days after December 12, which is 1 week and 6 days, so Saturday + 6 = Friday.
Fun facts about weekdays
- The Gregorian calendar repeats every 400 years, and in that cycle the 13th of the month falls on a Friday slightly more often than on any other weekday.
- A calendar from 2015 works again in 2026, because both are non-leap years starting on a Thursday.
- Dates before the Gregorian calendar (1582 in most Catholic countries, 1752 in Britain and its colonies) were recorded in the Julian calendar, so historical sources may differ from modern calculators.
Frequently asked questions
How can I find out what day I was born?
Enter your birth date in a day of the week calculator. It applies the Gregorian calendar rules and returns the weekday instantly.
What day of the week was January 1, 2000?
Saturday.
Why does my birthday move one day each year?
Because 365 days is 52 weeks plus one day. After a leap day, it moves two days instead.
Can I reuse an old calendar?
Yes, if the old year starts on the same weekday and has the same leap-year status. For example, the 2015 calendar matches 2026.
Lawyer for almost 20 years, focused on digital law, and creator of CoringaWeb. He writes the guides and builds the tools on this site.
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