Caelius has a nice post up on the Golden Number and calculations for Easter and such.
Kalendar arithmetic (the art of the computus) was an important part of the liturgical arts back in the day. Isidore includes astronomy in Book 3 on Mathematics along with music and geometry but puts his section on the Paschal Cycle in Book 6 where he talks about the book and services of the Church. (Here’s a handy fully hyperlinked table of contents for the whole Etymologiae.) Bede wrote two books on time, De Temporibus and the later De Temporum Ratione (see the table of contents here), that teach calendar calculations. The second is the more complete treatment.
Furthermore, this was an important enough matter that the two great English translators of things ecclesiastical into the vernacular—Ælfric and Bryhtferth—both tackled the topic. Indeed, Bryhtferth’s Enchiridion is theoretically a work focused on the calendar and computus but he meanders through all sorts of areas to get there. Ælfric’s De Temporibus Anni is far more lucid, drawing primarily from Bede and supplementing with Isidore.
Where the rubber really hits the road, though are the tables like those that begin on folio 45v of the Leofric Missal… And, hey, as long as you’re poking around those parts of that manuscript, check out the Christ and Satan pictures too.