As defined by the Catechism of the Catholic Church, the Liturgical Year is "The celebration throughout the year of the mysteries of the Lord's birth, life, death, and Resurrection in such a way that the entire year becomes a 'year of the Lord's grace'. Thus the cycle of the liturgical year and the great feasts constitute the basic rhythm of the Christian's life of prayer, with its focal point at Easter" (§1168).
The Catholic Church sets aside certain days and seasons of each year to recall and celebrate various events in the life of Christ. The Liturgical Calendar begins every year during the month of November on the First Sunday of Advent and runs through to the Solemnity of Christ the King.
The "Lectionary," the Mass readings from the Holy Bible, follows a Sunday cycle and a weekday cycle. The Liturgical Calendar follows a three year cycle, each year being represented by the letters, A, B and C.
During the year A cycle, the Gospel of Matthew is the primary Gospel that is used for the readings. In year B, Mark is the primary Gospel. In year C Luke is the primary Gospel. The Gospel of John is proclaimed on particular Sundays in each of the years.
On weekdays in Ordinary Time, there is a 2 year cycle numbered I and II. Year I is read in odd number years such as 2009, 2011, 2013. Year II is read in even years such as 2010, 2012, 2014.
If a person attends the Holy Mass everyday for 3 years, having been present for all the readings of the 3 cycles, most of the Holy Bible will have been read to him during that time frame.
In each cycle of the Liturgical Calendar, you will find six Seasons:
(5) Easter, and
(6) Ordinary Time.
During the year, in addition to the Sunday worship, the Church also celebrates Solemnities, Feasts, and Memorials which may be on any day of the week. These occur during the year to commemorate special events or persons that are highly revered by the Catholic Church.
In its Roman Rite the liturgical year begins with Advent, the time of preparation for both the celebration of Jesus' birth, and his expected second coming at the end of time. This season lasts until 24 December (Christmas Eve).
Christmastide follows, beginning with First Vespers of Christmas on the evening of 24 December and ending with the Feast of the Baptism of the Lord.
Lent is the period of purification and penance which begins on Ash Wednesday and ends on Holy Thursday.
The Holy Thursday evening Mass of the Lord's Supper marks the beginning of the Easter Triduum, which includes Good Friday, Holy Saturday, and Easter Sunday. These days recall Jesus' Last Supper with his disciples, death on the cross, burial, and resurrection.
The seven-week liturgical season of Easter immediately follows the Triduum, climaxing at Pentecost. This last feast recalls the descent of the Holy Spirit upon Jesus' disciples after the Ascension of Jesus.
The rest of the liturgical year is commonly known as Ordinary Time.