Why do Easter and Christmas form the two main cycles of the Christian church year?

Easter and Christmas serve as the two primary pillars of the Christian church year because they function as the controlling anchors for the "Proper of Time" (Temporale), the cycle of seasons and Sundays that structures the liturgical calendar.1

These two feasts represent the core of the Christian narrative:
* Easter: Centered on the Resurrection of Christ, it is considered the "feast of feasts" and serves as the absolute center of the liturgical year, particularly in Eastern traditions. Its movable date, determined by the lunar calendar, dictates the timing of related seasons like Lent and Pentecost.231
* Christmas: Marking the birth of Jesus, this festival celebrates the Incarnation—the event in which God became human—symbolizing divine love for humanity. It provides a fixed-date anchor that balances the liturgical year alongside the movable Easter cycle.41

Together, these cycles act as an epitome of the history of salvation, allowing the church to celebrate the total revelation and redemption of Christ throughout the year.1