Easter, not Christmas, is the structural center: Paul says flatly that if the resurrection did not happen, the faith is empty. Its date floats because it is calculated from the lunar Passover, and the Eastern and Western churches use different calendars, so they often celebrate weeks apart — a long-running ecumenical irritation. The observance is a three-day arc (Thursday's supper, Friday's crucifixion, Sunday's empty tomb), and the Orthodox tradition in particular treats the whole liturgical year as an orbit around it.