Clerical science, an offshoot of mathematics, dealing with calendars. Computistics (and its practitioners, computists) plays a major part in Christian calendar calculations, since many major Christian holy days are based on Easter. Easter not being set on a fixed date1, the Christian calendar is quite dependent upon diligent attention to the variations that govern Easter and determine the progress of the liturgical year.
1 - Originally, Christian Easter was held concurrently with Jewish Passover. Wishing to ensure that Easter always fell on a Sunday, the Council of Nicaea (325) set Easter as the Sunday following the first full moon of spring. Later modifications (attributed to Dionysius Exiguus, c. 525) set the date of Easter as the first Sunday following the first full moon on or after the vernal equinox. If the full moon is on a Sunday, then Easter Sunday is the following Sunday.