Why do Church seasons change?

The first words of the Universal Norms on the Liturgical Year and the Calendar, drawn from the Second Vatican Council’s Constitution on the Sacred Liturgy, summarize the profound meaning of the liturgical celebrations of the Church and their organization:

Holy Church celebrates the saving work of Christ on prescribed days in the course of the year with sacred
remembrance. Each week, on the day called the Lord’s Day, she commemorates the Resurrection of the Lord, which she also celebrates once a year in the great Paschal Solemnity, together with his blessed Passion. In fact, throughout the course of the year the Church unfolds the entire mystery of Christ and observes the birthdays of the Saints.

The liturgical year consists of a seasonal cycle and a sanctoral cycle, called the Proper of Time and the Proper of Saints, respectively. The Paschal Mystery of Jesus Christ—his suffering, death, and resurrection—is continuously proclaimed and renewed through celebrating the events of his life and in the feasts of the Blessed Virgin Mary and the saints.

Liturgical Year
The liturgical year is made up of six seasons:

 Advent—four weeks of preparation before the celebration of Jesus’ birth

 Christmas—recalling the Nativity of Jesus and his manifestation to the peoples of the world

 Lent—a six-week period of penance before Easter

 Sacred Paschal Triduum—the holiest “Three Days” of the Church’s year, where the Christian people recall the suffering, death, and resurrection of Jesus
– Holy Thursday: Mass of the Lord’s Supper
– Good Friday of the Lord’s Passion
– Easter: Mass of the Resurrection of the Lord

 Easter—50 days of joyful celebration of the Lord’s resurrection from the dead and his sending forth of the Holy Spirit

 Ordinary Time—divided into two sections (one span of 4-8 weeks after Christmas Time and another lasting about six months after Easter Time), wherein the faithful consider the fullness of Jesus’ teachings and works among his people

The mystery of Christ, unfolded through the cycle of the year, calls us to live his mystery in our own lives. This call is best illustrated in the lives of Mary and the saints, celebrated by the Church throughout the year. There is no tension between the mystery of Christ and the celebration of the saints, but rather a marvelous harmony.

(Source: United States Council of Catholic Bishops)

Read more in the “Why Do We Do That?” series from Deacon Mike Fritz.