Christmas Time and Easter Time highlight the central mysteries of the Paschal Mystery, namely, the incarnation, death on the cross, resurrection, and ascension of Jesus Christ, and the descent of the Holy Spirit at Pentecost. The Sundays and weeks of Ordinary Time, on the other hand, take us through the life of Christ. This is the time of conversion. This is living the life of Christ.
Ordinary Time is a time for growth and maturation, a time in which the mystery of Christ is called to penetrate ever more deeply into history until all things are finally caught up in Christ. The goal, toward which all of history is directed, is represented by the final Sunday in Ordinary Time, the Solemnity of Our Lord Jesus Christ, King of the Universe. (1)
Ordinary Time begins after the Christmas season with the feast of the Baptism of Jesus. It continues until the beginning of Lent on Ash Wednesday. Ordinary Time continues after Pentecost which closes the the Easter Season 50 days after Easter Sunday. Ordinary Time ends with the feast of Christ the King, usually in November. The following week begins the new Liturgical Season with the Advent Liturgical Season.
(1) Taken from US Conference of Catholic Bishops website.