How do lunar and solar calendars affect the dates of Christian feasts?

The Christian church year is structured around two concurrent cycles: the Proper of Time, which includes movable feasts dependent on Easter, and the Proper of Saints, which consists of fixed-date commemorations.1

The date of Easter is the primary factor determining the timing of other movable feasts in the church year. Because the Resurrection is historically linked to the Jewish Passover—a festival rooted in a lunar calendar—ecclesiastical authorities had to reconcile lunar phases with the solar calendar.2

In the Western church, this reconciliation resulted in the rule that Easter is celebrated on the first Sunday following the Paschal full moon that falls on or after the vernal equinox (fixed at March 21). This complex calculation requires balancing the lunar cycle—historically tracked using golden numbers or the epact—with the solar year to ensure the feast remains in its proper season. Consequently, while fixed feasts like Christmas remain on the same solar date, the movable feasts shift annually to maintain their relationship with the lunar-based timing of the Passover.21