The liturgical year begins with Evening Prayer I (traditionally called First Vespers) of the First Sunday of Advent. The season of Advent continues through the four Sundays of Advent and ends at Christmas Eve.
Advent is a time of preparation, through fasting and prayer, for Christmas. Even though Christ was actually born over 2000 years ago, during Advent we prepare our hearts to “receive” Jesus into the world each year as a light to the nations, at a time when our calendar is reaching its darkest period. Advent is also a time of looking forward to Christ’s Second Coming in the last days.
At Christmas we celebrate the Word become flesh, coming to dwell among us as the light of the human race, just after the darkest point of the solar year. Christmas, therefore, is a holy day second only to Easter in the Roman calendar.
The Octave of Christmas (octave means eight; hence the octave of Christmas lasts for eight days) begins with Christmas day and ends after the Solemnity of Mary, Mother of God.
The season of Christmas ends, and Ordinary Time begins, on the Monday after the Solemnity of the Baptism of the Lord, which signifies the purification of the world, through Christ Himself.
Ordinary Time derives its name from the word ordinal, meaning number. This season, therefore, is a season of weeks counted by numbers, from the First Week in Ordinary Time through the Thirty-Fourth Week in Ordinary Time. (Depending on the placement of Lent and Advent in any calendar year, Ordinary Time may end before the Thirty-Fourth Week.)
The liturgical season of Lent lasts for 40 weekdays in remembrance of the 40 days and nights that Christ spent fasting in the desert, tempted by Satan. The beginning of Lent, Ash Wednesday, therefore comes 40 days (excluding Sundays) before Easter.
Lent, in commemoration of Christ’s fasting and prayer, is for all His faithful a time of fasting and prayer.
Because of the austerity of Lent, Alleluia is not said in prayer or sung in liturgy during this season.
The season of Easter begins at the Easter Vigil.
But before that, the week previous to Easter is called Holy Week; it begins with Passion Sunday (Palm Sunday) and culminates with the Triduum. The Triduum (a Latin word for a three-day period) begins with the Mass of the Lord’s Supper on the evening of the Thursday of Holy Week and includes Good Friday, and Holy Saturday.
Easter is such a special time—the celebration of our Lord’s resurrection, without which there would be no Christianity—that it continues not just for the joyful week following Easter (the Octave of Easter—each day celebrated as a solemnity of the Lord), but for 50 days (including Sundays and counting Easter Sunday itself) of the season of Easter.
The season of Easter comes to a close, and Ordinary Time returns, on the Monday after Pentecost Sunday.