Day of Week Calculator
Find out what day of the week any date falls on — past or future
What Day Was That? The Algorithms Behind the Answer
Built and verified by Andrius R. · Updated June 2026
"What day of the week was 20 July 1969?" (a Sunday — Apollo 11's landing) sounds like it needs a lookup table covering all of history. It doesn't: the calendar is periodic enough that a few lines of arithmetic answer any date — and one famous method lets humans do it mentally at party speed.
Why it's computable at all
Weekdays shift by one per ordinary year (365 = 52 weeks + 1 day) and two per leap year — so the calendar's weekday pattern repeats on a fixed 400-year Gregorian cycle (exactly 20,871 weeks). Everything else is bookkeeping: month lengths and the leap rule (divisible by 4, except centuries, except divisible-by-400). Formal methods like Zeller's congruence (1882) compress that bookkeeping into one modular-arithmetic formula — the kind of expression this calculator evaluates instantly and spreadsheets hide behind WEEKDAY().
The Doomsday method: mental calendar arithmetic
In any year, an easily-memorized family of dates all fall on the same weekday — the year's "doomsday": 4/4, 6/6, 8/8, 10/10, 12/12, plus mnemonic friends 5/9, 9/5, 7/11, 11/7, Pi Day 3/14, and the last day of February.
For 2026 the doomsday is Saturday. So: 4 July 2026? July 11 is doomsday-Saturday → July 4 is exactly a week earlier → Saturday. Christmas 2026? December 12 is Saturday → +13 days → Friday. One memorized weekday per year unlocks the whole calendar; Conway practiced until he answered any date in under two seconds, and his computer greeted him with date quizzes at login.
Patterns that fall out of the math
- Your birthday advances one weekday per year, two after a leap day — and returns to its birth-weekday on the famous 5-6-11-or-28-year rhythm.
- The 13th is most often a Friday. Genuinely: across the 400-year cycle the 13th lands on Friday 688 times — more than any other weekday (Thursday and Saturday: 684). The calendar is structurally, mildly, ominous.
- Centuries are weekday-biased: under Gregorian rules, the 1st of a century year can only be Monday, Tuesday, Thursday or Saturday — no century will ever begin on a Sunday, Wednesday or Friday.
- Identical calendar years recur: 2026's calendar (starts Thursday, non-leap) is reusable in 2037 — vintage calendars from matching years are functionally current, a fact that delights both collectors and the thrifty.
The historical fine print
Two caveats keep historical answers honest. First, the calendar switch: before a country adopted the Gregorian reform (1582 in Catholic Europe, 1752 in Britain and its colonies, 1918 in Russia), documents used Julian dates — so "what day was 23 April 1616" has different answers for Shakespeare's England (Julian) and Cervantes' Spain (Gregorian), and indeed the two writers died on the same nominal date but ten real days apart. This calculator, like virtually all software, extends today's Gregorian rules backwards (the "proleptic" calendar) — consistent, but cross-check against the local calendar for pre-adoption history. Second, the week itself never broke: through every calendar reform, the seven-day cycle ran uninterrupted — when Britain deleted 11 days in 1752, Wednesday 2 September was simply followed by Thursday 14 September. The dates jumped; the weekdays didn't miss a beat.
// Doomsday Algorithm
John Conway's Doomsday algorithm lets you mentally calculate the day of week for any date. Key anchor days: 4/4, 6/6, 8/8, 10/10, 12/12 all fall on the same weekday each year.
// ISO Week Number
ISO week 1 is the week containing the first Thursday of January. Some years have 53 weeks. Week numbers are used in European business contexts.
// Friday 13th
Every year has at least one Friday the 13th. The most in one year is three (when Jan 1 falls on Thursday in a non-leap year).
// Born on a Monday
"Monday's child is fair of face…" — the old nursery rhyme assigns personality traits to birth weekdays. About 1 in 7 people share any given birth weekday.