“We must continue to accomplish in ourselves the stages of Jesus’ life and his mysteries . . . For it is the plan of the Son of God to make us and the whole Church partake in his mysteries” (#521, quoting St. John Eudes).
These words come from a sermon by St. John Eudes, who is quoted in the Catholic Catechism. The words express well why we celebrate the Church's year, also called the liturgical year.
The central day of the Church's year is Easter, the resurrection of Jesus from the dead. Our Sundays lead up to that as we begin the liturgical year at the end of November with the Advent Season. Advent is four weeks and concludes on December 24. Catholics take the four weeks to prepare to celebrate Christmas -- which is more than just a "birthday." At Christmas we are proclaiming that in Jesus God entered the human race, in order to prove God's love for each of us. This mystery is called the Incarnation.
The Christmas Season is rather short. We celebrate Mary's motherhood, and the Magi who represent that Christ is for the whole world, and the Baptism of Jesus.
Lent begins in February -- with Ash Wednesday -- and includes six Sundays. Lent's last 3 days are called the Sacred Triduum, Holy Thursday, Good Friday and Holy Saturday. The great gift we seek in Lent is God's grace so that our ongoing Conversion may continue -- that we may "accomplish in our lives the stages of Jesus' life."
The Easter Season begins on Easter Sunday and lasts for fifty days, concluding on Pentecost when we celebrate the daily presence and help of the Holy Spirit.
Between Pentecost and the next Advent Season we celebrate about 24 weeks of "Ordinary Time." On Sundays and weekdays the Scriptures feed us, so that we can welcome the Lord into our lives each day, especially by loving our neighbor.