SUMS: Gruenewald -- Calculating the date of Easter

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).