📆 Day of the Week Calculator
How It Works & How Accurate It Is
Click the field to pick a date from the calendar (or type YYYY-MM-DD) and hit Look Up. The result comes from standard Gregorian calendar arithmetic with full leap-year handling, so any past or future date is accurate — July 20, 1969 (the Apollo 11 landing), for instance, returns Sunday.
One caveat for historians: before October 15, 1582 the Gregorian calendar didn't exist, and Britain and its colonies kept the Julian calendar until 1752. George Washington's birthday was recorded as February 11, 1731 (Old Style) but is celebrated as February 22, 1732 — always check which calendar a historical document uses before looking its dates up here.
The Doomsday Rule: Weekdays in Your Head
Mathematician John Conway devised a famous trick for computing weekdays mentally. It rests on one fact: every year, a fixed set of easy-to-remember dates all fall on the same weekday — the year's "doomsday."
- Memorize the anchor dates: 4/4, 6/6, 8/8, 10/10, 12/12 (even months doubled), plus 5/9, 9/5, 7/11, 11/7 (the mnemonic: "9-to-5 at the 7-Eleven"), 3/14 (Pi Day), and the last day of February (Feb 28, or Feb 29 in leap years). All of them share the year's doomsday.
- Find the year's doomsday: for 2000–2099, compute
(2 + Y + ⌊Y/4⌋) mod 7where Y is the two-digit year; 0=Sunday, 1=Monday, … 6=Saturday. - Count from the nearest anchor: however many days your target date is from an anchor, shift the weekday by the same amount.
Worked example — July 4, 2025: Y=25, so (2 + 25 + 6) mod 7 = 33 mod 7 = 5 → doomsday 2025 is Friday. July's anchor is 7/11 (a Friday), and 7/4 is exactly seven days earlier — so July 4, 2025 is also a Friday, matching the tool.
Weekday Numbers and ISO 8601 Weeks
Software and paper calendars disagree about how weeks work, which causes real bugs and scheduling mix-ups:
- Week start: ISO 8601 (used across Europe and in most programming standards) defines Monday as day 1 and Sunday as day 7; US calendars traditionally start the week on Sunday.
- Week numbers: ISO week 1 is the week containing the year's first Thursday — so January 1 can belong to week 52/53 of the previous year. If a colleague says "week 14," confirm whether they mean ISO weeks.
- In spreadsheets: Excel's
WEEKDAY()returns 1=Sunday by default; pass 2 as the second argument for ISO-style 1=Monday.ISOWEEKNUM()gives the ISO week number.
Frequently Asked Questions
How far back or forward can I look up?
Any date the Gregorian calendar can express, from year 1 to far in the future. Just remember the Julian-calendar caveat above for dates recorded before a region adopted the Gregorian calendar.
Do leap years change the answer?
Yes — the extra February 29 shifts every date from March onward by one weekday relative to a non-leap year. That's exactly why the Doomsday rule uses "last day of February" as an anchor: it absorbs the leap-day shift automatically. The tool handles all of this for you.
Why does the same date drift by one or two weekdays each year?
A common year is 365 days = 52 weeks + 1 day, so any fixed date moves forward one weekday per year — and two after a leap day passes. Your birthday cycles through the whole week roughly every 5–6 years for this reason.
What input format does the tool accept?
Use the calendar picker, or type YYYY-MM-DD (e.g. 2025-07-04) and press Enter. The unambiguous ISO format avoids the classic MM/DD vs DD/MM confusion between US and European conventions.