Why any year can be worked out
The Gregorian calendar hides no irregularities: it is an arithmetic rule that repeats. Months always have the same number of days except February, and February depends on a single condition, the leap year one. With those two facts, and knowing the weekday of any one reference date, the calendar of any year can be rebuilt without looking anything up.
The leap year rule has three layers, and hardly anyone remembers the third: every multiple of four is a leap year, except multiples of one hundred, except multiples of four hundred. It exists because the solar year lasts 365 days and almost six hours, but not exactly: eleven minutes a year are left over, and the two exceptions correct them. Without them the calendar would drift a day every 128 years, which is precisely what happened to the Julian one and why ten days had to be skipped in 1582.
How to find the day of the week in your head
The quickest mental method is John Conway's doomsday rule. It rests on the fact that every year has one weekday — the “doomsday” — on which a set of easily remembered dates all fall: 4/4, 6/6, 8/8, 10/10 and 12/12, plus the last day of February.
- Find the doomsday of the year: in 2026 it is Saturday.
- Take the anchor date of your month: for August that is 8 August.
- Count the days from that anchor to the date you want.
- Move that many days forward through the week from the doomsday.
With practice it takes a few seconds, and it works for any date in any century. The calculator above does the same thing, only it draws the whole month.
What is worth knowing
A calendar repeats every 6, 11 or 28 years, depending on how many leap years fall in between. The full cycle is 400 years: after that the Gregorian calendar starts over exactly as before, with the same weekdays on the same dates. Among other things, that means the 13th falling on a Tuesday or a Friday has a fixed, computable frequency — and that Friday the 13th is, by a narrow margin, the most common 13th of all.
Two warnings about old dates. First: the Gregorian calendar came into force in October 1582, and that month ten days were skipped in one go, from the 4th to the 15th. For earlier dates this calculator still applies the Gregorian rule backwards — what is called the proleptic calendar, the ISO standard's criterion — which does not match what was recorded at the time. Second: countries did not adopt it together. England and its colonies waited until 1752 and Russia until 1918, so dates in documents from those centuries have to be read knowing which calendar the writer was using.