Day of the Week Calculator
Find the day of the week for any date — past or future. You also get the day-of-year number, ISO week, leap-year status, and how many days away it is, plus a walkthrough of the “doomsday” trick for working out weekdays in your head.
Dates use the Gregorian calendar (extended backwards before 1582 as a “proleptic” calendar — see the FAQ).
How the weekday of a date is found
Weekdays cycle every 7 days, so any date’s weekday follows from how many days it sits from a known anchor. A regular year is 365 days = 52 weeks + 1 day, which is why your birthday advances one weekday most years and two after a leap day. Calendars repeat exactly every 400 years (146,097 days — conveniently divisible by 7), so 2026’s calendar is identical to 1626’s.
The famous mental shortcut is John Conway’s doomsday rule: in any year, the dates 4/4, 6/6, 8/8, 10/10, 12/12, plus 5/9, 9/5, 7/11, 11/7, 3/14, the last day of February, and 1/3 (1/4 in leap years) all fall on the same weekday — that year’s “doomsday.” Find the doomsday, then step to your date.
How it’s calculated
Weekday = (days since a known epoch) mod 7, computed in UTC. Day of year counts from January 1; ISO-8601 week numbers start on Monday, with week 1 containing the year’s first Thursday. Leap years: divisible by 4, except centuries not divisible by 400.
Historical dates before a country adopted the Gregorian calendar (1582 in Catholic Europe, 1752 in Britain/America) were recorded in the Julian calendar and can differ by 10–11 days.
Doomsday method cheat sheet
| Month | Doomsday date | Memory hook |
|---|---|---|
| Jan | 1/3 (1/4 leap) | 3rd for 3 common years, 4th in the 4th |
| Feb | 2/28 (2/29 leap) | Last day of February |
| Mar | 3/14 | Pi day |
| Apr–Dec even | 4/4, 6/6, 8/8, 10/10, 12/12 | Doubles |
| Odd months | 5/9, 9/5, 7/11, 11/7 | “9-to-5 at the 7-11” |
Century anchor days: 1900s → Wednesday, 2000s → Tuesday, 2100s → Sunday, 2200s → Friday (the pattern repeats every 400 years).
Worked example
Take July 20, 1969 (the Apollo 11 moon landing). Doomsday math: the 1900s anchor is Wednesday (3). 69 ÷ 12 = 5 remainder 9, and 9 ÷ 4 = 2, so 3 + 5 + 9 + 2 = 19 → 19 mod 7 = 5 = Friday is 1969’s doomsday. July’s doomsday date is 7/11 (a Friday), and July 20 is 9 days later: 9 mod 7 = +2 → Sunday. The calculator agrees — and shows it was day 201 of 1969, ISO week 29, in a non-leap year.
Common mistakes
- Forgetting the leap-year exception: 1900 wasn’t a leap year, but 2000 was (divisible by 400).
- Using January or February doomsdays without the leap-year adjustment (1/4 and 2/29 in leap years).
- Applying the Gregorian calendar to very old dates — George Washington’s birthday moved 11 days when Britain switched in 1752.
- Confusing ISO week numbers (Monday start, week 1 holds the first Thursday) with U.S.-style Sunday-start weeks.
Where it is used
- Finding the weekday you, a parent, or a grandparent was born.
- Checking which weekday a future birthday, anniversary, or deadline lands on.
- History and genealogy research on documented dates.
- Practicing the doomsday mental-math trick.
Frequently asked questions
What weekday was I born?
Enter your birth date and read the top line. Fun check: your birthday weekday advances one day most years and two in years right after a leap day, returning to the original weekday on a 28-year cycle (for most of your life).
What is the doomsday rule?
John Conway’s trick: memorable dates like 4/4, 6/6, 8/8, 10/10, 12/12, 5/9, 9/5, 7/11, 11/7 and 3/14 all share one weekday each year. Compute that “doomsday” from the year, then count forward or back a few days to your target date.
How accurate is this for historical dates?
The math extends the modern Gregorian calendar backwards indefinitely (“proleptic”). It matches recorded history after a country adopted the Gregorian calendar — 1582 in much of Europe, 1752 in Britain and colonial America. Earlier records used the Julian calendar, offset by 10–11 days.
What is an ISO week number?
ISO-8601 weeks start on Monday and week 1 is the week containing the year’s first Thursday, so a year has 52 or 53 numbered weeks. Businesses across Europe schedule by these numbers.
Why do the same calendars repeat?
The Gregorian calendar cycles every 400 years — exactly 146,097 days, which is divisible by 7. Shorter matches happen too: a common year starting on the same weekday repeats after 6 or 11 years, which is why 2025 and 2031 share a calendar.