Day of the Week Calculator
What day of the week were you born? What day does a future date land on? Enter any date — historical or far future — and get its weekday instantly, plus its position in the year.
How to use this calculator
- Pick any date — your birth date, a historical event, a future deadline.
- Press the button to see its weekday and its position in the year.
- Dates centuries away work fine — the calendar rules are applied exactly.
Formula used
Computers count days from a fixed epoch and take the remainder mod 7. By hand, mathematicians use Zeller's congruence or the Doomsday rule — John Conway's trick exploiting that 4/4, 6/6, 8/8, 10/10 and 12/12 always share a weekday within any year.
Example calculation
1 January 2000 was a Saturday — day 1 of a leap year with 366 days.
Party trick: in every year, 4 April, 6 June, 8 August, 10 October and 12 December all fall on the same weekday (the year's "doomsday"). Verify it above.
Finding weekdays without a calendar
The weekday of any date is fully determined by calendar arithmetic: years shift the weekday by one (two after a leap day), and the Gregorian pattern repeats every 400 years. That regularity powers mental-math methods like the Doomsday rule, with which practiced people name the weekday of any date in seconds.
Practically, the question comes up for birth certificates ("born on a Sunday"), historical research, and planning — knowing a future date is a Monday changes what you book around it.
Why use this calculator?
- Settle "what day was I born?" definitively.
- Check which weekday future holidays, deadlines and events land on.
- Verify historical dates for research, anniversaries and documents.
Frequently asked questions
What day of the week was I born?
Enter your date of birth above. Fun fact: your birthday's weekday advances by one each year — two when a leap day intervenes — which is why it cycles through all seven days over 5–11 years.
Does this work for very old dates?
It applies the modern (Gregorian) calendar rules to all dates. For events before a country adopted the Gregorian calendar (1582 in Catholic Europe, 1752 in Britain, later elsewhere), historical records may use the Julian calendar, which differs by several days.
What is the Doomsday rule?
A mental algorithm by John Conway: certain easy dates (4/4, 6/6, 8/8, 10/10, 12/12, plus the last day of February) always share one weekday in a given year. Learn that year's "doomsday" and you can derive any date's weekday with small offsets.