What Day of the Week Was (or Will Be) a Date?

How this calculator works

The calendar repeats a fixed pattern, so the weekday of any date can be worked out exactly. The calculator uses your browser’s calendar (the proleptic Gregorian calendar), and the “Show my working” list walks through John Conway’s Doomsday method — the trick for finding a weekday in your head.

The idea: in every year, certain easy dates — 4/4, 6/6, 8/8, 10/10, 12/12, 5/9, 9/5, 7/11, 11/7 and the last day of February — all fall on the same weekday, called that year’s “doomsday.” Find the doomsday, then count from the nearest one to your date.

Assumptions and tips

• Uses the Gregorian calendar for all dates, including those before it was adopted (1582 in Catholic countries, 1752 in Britain and its colonies).

Worked example

Using the default values in the calculator above:

Step by step:

1. Century anchor for 1900s: Wednesday
2. Year part: 90 ÷ 12 = 7 remainder 6; 6 ÷ 4 = 1
3. Doomsday 1990 = (3 + 7 + 6 + 1) mod 7 = 3 → Wednesday
4. Nearest doomsday date in month 7: day 11
5. 15 − 11 = 4 days; Wednesday + 4 days → Sunday

What day is Christmas?

Christmas Day for the next few years, from this calculator.

Common mistakes to avoid

• Forgetting the leap-year exception for January and February in the Doomsday method.
• Using the Gregorian calendar for old dates in countries that still used the Julian calendar at the time.

Frequently asked questions

Sources

• Conway’s Doomsday rule (method used in the step-by-step working)

Built and reviewed by Anirban Kundu, Creator & Editor · M.Sc Physics, 20 years teaching science

Last reviewed: 2026-11-05 · Editorial policy · How we build and check calculators