What day of the week was it?
Pick any date between 1600 and 2400 and the weekday appears in big letters: the day you were born, the date of a historic event, a wedding next summer or a contract deadline. The sentence follows the tense for you: a past date “was” a Thursday, a future one “will be” a Friday.
Under the weekday you also get:
- The day of the year, such as day 306 of 365. Leap years have 366 days.
- The week number, in the US system used on American calendars, where weeks start on Sunday. For ISO weeks, see the week number tool.
- The distance from today, in days.
- The same date in the next years, handy to see which weekday a birthday or an anniversary will fall on.
Why a date changes weekday every year
A common year has 365 days, which is 52 weeks and 1 day. So each date moves one weekday later from one year to the next: November 2 was a Sunday in 2025 and is a Monday in 2026. After a February 29, the shift is two days. That is why a calendar can only be reused after 6, 11 or 28 years, and why the whole Gregorian calendar repeats exactly every 400 years: 400 years hold 146,097 days, which is exactly 20,871 weeks.
Work it out in your head: the Doomsday method
The mathematician John Conway found a trick to find any weekday mentally. It rests on one fact: in a given year, a set of easy dates always fall on the same weekday, called the year’s “doomsday”.
| Month | Doomsday date | Memory hook |
|---|---|---|
| January | 3 (or 4 in a leap year) | 3 years out of 4, the 3rd |
| February | Last day (28 or 29) | The end of the month |
| March | 14 | Pi Day, 3/14 |
| April, June, August, October, December | 4/4, 6/6, 8/8, 10/10, 12/12 | Even months: same number |
| May and September | 5/9 and 9/5 | “9 to 5” |
| July and November | 7/11 and 11/7 | “at 7-Eleven” |
To find the year’s doomsday, take the last two digits of the year (YY) and the century’s anchor day:
- Divide YY by 12: keep the whole number (a) and the remainder (b).
- Divide b by 4 and keep the whole number (c).
- Add a + b + c to the century anchor, counting weekdays forward. Drop whole weeks.
| Years | Anchor day |
|---|---|
| 1700 to 1799 | Sunday |
| 1800 to 1899 | Friday |
| 1900 to 1999 | Wednesday |
| 2000 to 2099 | Tuesday |
| 2100 to 2199 | Sunday |
Example: November 2, 2026. YY = 26. 26 ÷ 12 = 2, remainder 2; 2 ÷ 4 = 0. So a + b + c = 4. The 2000s anchor is Tuesday, and four days after Tuesday is Saturday: in 2026, doomsday is a Saturday. November 7 (11/7) is therefore a Saturday, and November 2, five days earlier, is a Monday.
Example: July 4, 1776. YY = 76. 76 ÷ 12 = 6, remainder 4; 4 ÷ 4 = 1. 6 + 4 + 1 = 11, which is one week and 4 days. The 1700s anchor is Sunday, plus 4 days: Thursday. July 11 (7/11) was a Thursday, so July 4, one week earlier, was a Thursday too.
The weekday in a spreadsheet
Excel and Google Sheets can do the same job for a whole column of dates. =TEXT(A1, "dddd") writes the weekday name of the date in A1, and "ddd" gives the short form (Mon, Tue). =WEEKDAY(A1) returns a number from 1 for Sunday to 7 for Saturday; =WEEKDAY(A1, 2) counts from 1 for Monday instead, which is handy to filter out weekends with a test such as WEEKDAY(A1, 2) > 5.
Dates before the calendar change
This calculator uses the Gregorian calendar, the one in use today, for every date. It was introduced in 1582: in Spain, Portugal and most of Italy, Thursday, October 4, 1582 was followed by Friday, October 15. Other countries switched later. Great Britain and its American colonies only changed in 1752, when Wednesday, September 2 was followed by Thursday, September 14. For a date recorded in the old Julian calendar, such as an English or colonial record before September 1752, the weekday in old documents can differ from the one shown here.
Where the names of the days come from
The seven-day week reached English through Latin and the old Germanic gods. Sunday and Monday are the days of the Sun and the Moon, and Saturday is Saturn’s day. The four others honor Norse and Germanic gods: Tuesday for Tiw, a god of war, Wednesday for Woden (Odin), Thursday for Thor and Friday for Frigg. Romance languages kept the Roman planets instead, which is why Tuesday is mardi (Mars) in French and Friday is viernes (Venus) in Spanish.
Which day comes first depends on where you are. American calendars start the week on Sunday; the ISO 8601 standard and most European calendars start it on Monday, which changes how weeks are numbered.
Friday the 13th and other curiosities
Every year has at least one Friday the 13th and at most three. In 2026, it happens in February, March and November. Over the full 400-year cycle, the 13th of the month falls on a Friday slightly more often than on any other weekday: 688 times, against 684 to 687 for the others. And Christmas 2026 falls on a Friday.
To count the days between two dates, use the date calculator. To find your exact age and the weekday of your next birthday, try the age calculator, and for whole months at a glance, the calendar.