The joyful expectation of Advent finds its culmination in the second season of the liturgical year: Christmas. Traditionally, the Christmas season extended from First Vespers (or evening prayer) of Christmas (before Midnight Mass) through Candlemas, the Feast of the Presentation of the Lord (February 2)—a period of 40 days. With the revision of the calendar in 1969, "The Christmas season runs," notes the General Norms, "from evening prayer I of Christmas until the Sunday after Epiphany or after 6 January, inclusive"—that is, until the Feast of the Baptism of the Lord. Contrary to popular celebration, the Christmas season does not encompass Advent, nor end with Christmas Day, but begins after Advent ends and extends into the New Year. The season is celebrated with a special joy throughout the Twelve Days of Christmas, ending with the Epiphany of Our Lord (January 6).
The season of Ordinary Time is interrupted by three seasons, the first being Lent, the 40-day period of preparation for Easter. In any given year, the length of the first period of Ordinary Time depends on the date of Ash Wednesday, which itself depends on the date of Easter. Lent is a period of fasting, abstinence, prayer, and almsgiving—all to prepare ourselves, body and soul, to die with Christ on Good Friday so that we may rise again with Him on Easter Sunday. During Lent, the emphasis in the Mass readings and daily prayers of the Church is on the prophecies and foreshadowings of Christ in the Old Testament, and the increasing revelation of the nature of Christ and His mission
From Latin: “Three Days”, the Easter Triduum is the shortest; as the General Norms note, "The Easter Triduum begins with the evening Mass of the Lord's Supper (on Holy Thursday), continues with Good Friday, reaches its high point in the Easter Vigil (Holy Saturday), and closes with evening prayer on Easter Sunday." While the Easter Triduum is liturgically a separate season from Lent, it remains a part of the 40-day Lenten fast, which extends from Ash Wednesday through Holy Saturday, excluding the six Sundays in Lent, which are never days of fasting.
After Lent and the Easter Triduum, the third season to interrupt Ordinary Time is the Easter season itself. Beginning on Easter Sunday and running to Pentecost Sunday, a period of 50 days (inclusive), the Easter season is second only to Ordinary Time in length. Easter is the greatest feast in the Christian calendar, for "if Christ is not risen, our faith is in vain." The Resurrection of Christ culminates in His Ascension into Heaven (40 days after the resurrection) and the descent of the Holy Spirit on Pentecost (50 days after resurrection), which inaugurates the mission of the Church to spread the Good News of salvation to all the world