Day of the Week Calculator
Given any calendar date in YYYY-MM-DD form, return the day of the week (Monday through Sunday) under the Gregorian calendar, for historical dates as well as dates far in the future.
Last updated: September 2026
Day of the Week
Thursday
Formula below · 2 sources (NIST, aa.usno.navy.mil) · Updated Sep 2026
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About this calculator
Every date maps to exactly one day of the week. Modern calendars compute it with modular arithmetic such as Zeller's congruence (1882); this calculator reads the year, month and day you typed and asks the JavaScript Date object, which applies the Gregorian rules exactly: leap years every four years, except century years, except those divisible by 400. It works for any four-digit year, past or future, and rejects impossible dates such as 2026-02-29.
Julian-calendar dates (before 1582 in Catholic countries, before 1752 in Britain and its colonies, up to 1918 in Russia) follow a different rule set; for those the result is the proleptic Gregorian weekday, which can differ from the day printed on a calendar of the time.
Common uses: finding which weekday your birthday falls on this year, checking whether a deadline lands on a weekend, or verifying a historical event's date-day pair for a citation.
How to use
Example — New Year's Day 2026. Enter 2026-01-01. Result: Thursday. Verify: 2026 is not a leap year, and January 1, 2026 is confirmed as a Thursday by any standard calendar. Example — a historical date. Enter 1969-07-20 (the Apollo 11 moon landing). Result: Sunday. That matches the historical record — the landing took place on Sunday, July 20, 1969, at 20:17 UTC.
Frequently asked questions
Which day is 'first' — Sunday or Monday?
This calculator names the day (Monday, Tuesday, etc.) rather than numbering it, so the ISO-vs-US disagreement does not matter. If you need a number: ISO 8601 numbers Monday = 1 through Sunday = 7; the US convention starts weeks on Sunday and numbers Sunday = 1. Both conventions agree that a date's weekday is the same day.
Does it handle leap years?
Yes. February 29 falls on a valid weekday in leap years (2024, 2028, 2032, 2036…) and is rejected in non-leap years. The Gregorian rule is: divisible by 4, except centuries, except centuries divisible by 400. 2000 was a leap year; 2100 will not be.
What about Julian calendar dates?
Dates before the switch from Julian to Gregorian (1582 in Catholic Europe, later elsewhere — 1752 in the UK, 1918 in Russia) need a different rule set. JavaScript's Date always applies Gregorian rules, so results for Julian-era dates are 'the Gregorian equivalent' of that day, which differs from the day printed on a Julian calendar of the era.
Why does the tool sometimes disagree with a printed calendar?
The most common cause is time-zone parsing: a bare date can be read as midnight UTC or as local midnight depending on its format, which shifts it by a day for viewers far from UTC. This calculator reads the year, month and day you typed as a calendar date (no time zone) and takes its weekday, so results are the same wherever you are.
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Sources & references
Think a result is wrong? Report a problem with this calculator.