Happy Easter 2026!
In broad terms, Easter is the first Sunday after the first full moon after the spring equinox. Except when it isn’t.
The detail is more convoluted: Easter’s placement is based on the Jewish calendar; the date of Jesus being crucified is placed as the day of preparation for Passover, which occurs part way into the first month of the Hebrew year.
Everything that follows is then about how to represent that date in a more modern calendar, while considering that the Hebrew year is 354 days and adjusts periodically to add in a further month to realign with the 365 days of the solar year.
Easter is based on the Sunday after the first full moon - as determined by 14 days into the lunar month - after 21 March. The actual spring equinox might occur a day or two before, but the Gregorian calendar fixed it in place as 21 March as a simplification.
The Orthodox church, however, did not adopt the Gregorian calendar, and instead calculates based on the Julian calendar, which treats leap days slightly differently, putting the calendar 13 days out. As a result, the ‘first full moon after 21 March’ isn’t always the same full moon between the two calendars, so sometimes Easter is the same day between the two, sometimes it’s up to 5 weeks out.