Day of the Week Calculator

What You Can Look Up

Enter any date from the year 1 to 9999. The calculator shows the weekday, a small calendar of that month, how many days ago or ahead the date is, the ISO week number, the quarter, and the weekday of the same date one year later. It is handy for planning events, checking dates in documents, or answering questions about history. For your own birth date, the What Day Was I Born? calculator adds your age and next birthday.

Working Out the Weekday in Your Head

Mathematician John Conway’s Doomsday rule lets you find the weekday of any date mentally. It rests on the fact that certain easy-to-remember dates always share a weekday within a year — that year’s “doomsday”:

• 4/4, 6/6, 8/8, 10/10 and 12/12 (April 4, June 6, August 8, October 10, December 12).
• May 9, September 5, July 11 and November 7 — remembered as “9 to 5 at the 7-11”.
• March 14, and the last day of February.
• January 3 in common years, January 4 in leap years.

To find the doomsday for a year:

1. Start from the century’s anchor day: Friday for the 1800s, Wednesday for the 1900s, Tuesday for the 2000s.
2. Take the last two digits of the year. Add how many 12s fit into it, the remainder, and how many 4s fit into that remainder.
3. Count that many days forward from the anchor, ignoring whole weeks.

Example — March 14, 1998. The 1900s anchor is Wednesday. For 98: 98 ÷ 12 = 8 remainder 2, and 2 ÷ 4 = 0. That gives 8 + 2 + 0 = 10, which is 3 after removing a week. Three days after Wednesday is Saturday, so every doomsday in 1998 — including March 14 — was a Saturday.

How the Calendar Repeats

The Gregorian calendar repeats exactly every 400 years, because 400 years contain exactly 146,097 days — 20,871 weeks. Within the years 1901–2099, where every fourth year is a leap year without exception, the calendar repeats every 28 years: 1998 and 2026 have identical calendars, for example.

A calendar year can also be reused sooner. Any two common years that start on the same weekday share every date’s weekday, so the 2015 calendar also works for 2026.

Historical Dates

The Gregorian calendar was introduced in October 1582, when ten days were dropped to realign the calendar with the seasons. It was adopted at different times: Britain and its American colonies switched in September 1752, Russia in 1918, Greece in 1923. This calculator applies the Gregorian calendar to all dates, so for dates recorded in the older Julian calendar the weekday it shows may not match the original document.