The liturgical calendar (see Appendix A) is a way of making time, an essential part of creation, sacred. This highly developed structure that encompasses the entire year did not exist in the early Church. Sunday, the day for celebration of the Eucharist, contained the essential elements of the entire year — the Passion, Death, and Resurrection — so the “paschal festival” was renewed every Sunday. On the annual anniversary, though, the day would be celebrated with great solemnity, and eventually Easter became the focal point of the liturgical year. The feast of Easter was clearly linked to that of the Pentecost, the festival celebrating the Descent of the Holy Spirit on the apostles, fifty days after Easter.

Today, some feast days are fixed while others are based on seasonal changes and moon phases. For instance, the date for Easter is linked to the spring equinox. Every year, the date changes — Easter Sunday always occurs on the Sunday after the full moon following the equinox. Christmas, however, is fixed on December 25. This means that the liturgical calendar fluctuates from year to year.

Furthermore, the Church year consists of two distinct cycles, the temporal cycle and the sanctoral cycle. The temporal cycle is a series of solemn events celebrating the mystery of Christ — Advent, Christmas, Lent, Holy Week, Easter — divided into two cycles (the Christmas cycle and the Easter cycle), plus what the Church calls ordinary time, or the remainder of the year. The sanctoral cycle includes all the saints' feast days and many of the Marian feast days.