Doomsday Algorithm Weekday Calculator

About this calculator

The doomsday algorithm, popularized by mathematician John Conway, is a mental-math method for finding the day of the week for any date in history or the future. It works by memorizing each year's “doomsday” — a fixed weekday that several easy-to-remember dates (like 4/4, 6/6, 8/8, 10/10, and 12/12) always fall on — and then counting forward or backward from the nearest of those anchor dates to reach the target date.

This calculator applies the same logic programmatically: given any date, it derives that year's doomsday weekday, counts the offset to the requested date, and also reports the day-of-year number. It's popular with recreational mathematicians, competitive mental calculators, and calendar enthusiasts who use it as a party trick, as well as anyone who just wants to double-check what weekday a historical date or future deadline falls on without opening a calendar app.