What Is a Perpetual Calendar?
A perpetual calendar shows the day of the week for any date across many years, instead of just one. Pick any date above, from year 1 to 9999, to see what day of the week it falls on, browse the full month around it, and find the other years that share exactly the same calendar.
Years With the Same Calendar
Two years share a calendar when they start on the same day of the week and are both leap years or both common years. Between 1901 and 2099 the whole calendar repeats every 28 years, and common years often repeat after 6 or 11 years in between. The 2026 calendar, which starts on a Thursday, is the same as in these years:
1903, 1914, 1925, 1931, 1942, 1953, 1959, 1970, 1981, 1987, 1998, 2009, 2015, 2037, 2043, 2054, 2065, 2071, 2082, 2093, 2099
So a 2015 calendar works again in 2026, and a 2026 calendar can be reused in 2037.
How to Work Out the Day of the Week in Your Head
Mathematician John Conway's Doomsday rule makes it possible. In any year, certain easy-to-remember dates always fall on the same day of the week, called that year's doomsday: 4/4, 6/6, 8/8, 10/10 and 12/12, as well as 5/9, 9/5, 7/11 and 11/7, March 14 (Pi Day), and the last day of February. In 2026 the doomsday is a Saturday, so April 4, June 6 and December 12, 2026 are all Saturdays. From the nearest doomsday you count forward or back to your date.
To find a year's doomsday, start from the century's anchor day (Tuesday for 2000 to 2099, Wednesday for 1900 to 1999). Take the last two digits of the year, add how many times 12 goes into them, the remainder, and how many times 4 goes into that remainder, then count that many days forward from the anchor. For 2026: 26 contains two 12s with 2 left over, and 4 goes into 2 zero times, so 2 + 2 + 0 = 4 days after Tuesday, which is Saturday.
A Note on Historical Dates
This calendar uses the Gregorian calendar for all dates, extended backward before it was introduced in 1582 (the proleptic Gregorian calendar). Many countries switched later, including Britain and its colonies in 1752 and Russia in 1918, so dates recorded in the older Julian calendar in those places can differ by 10 to 13 days.
