This week’s very topical SUMS talk is being given by David Gruenewald. Pizza and drinks will be available afterwards. Abstract: The date of Easter (Sunday) changes from year to year. In fact part of the development of the modern-day (Gregorian) calendar was because the date of Easter seemed to move out of season too much. We shall study the date of Easter from a mathematical perspective. Can Easter be on the same date in consecutive years? Will the movable date of Easter ever coincide with the fixed date of Christmas (from a purely mathematical perspective)? Like all interesting questions, the answer is yes and no. All shall be revealed on "the day before the day before the day before Easter" (a phrase I was considering to use in an alternative talk title for my Thursday talk).