It consists of the cycle that determines when different Seasons of the Church, holy days, feast days, including celebration of Saints, are observed, and which portions of Scripture are to be read either in an annual cycle or in a cycle of several years at Mass for the Gospel and other readings.
Aside from the readings, the Liturgical Calendar also determines the interior decoration of a Church, the Priest's vestment colours (distinct liturgical colours are used in connection with different Seasons), the timing of spiritual seasons and practices such as Lent, and much more.
The Year is divided into seven main parts. The shortest but most holy being the Sacred Pascal Triduum. (The three days leading up to Easter.)
The Churches year starts with Advent in November or December and ends the following November or December with the Feast of Christ the King.