The word ordinary does not mean usual or every day but comes from a Latin word meaning "to order" (e.g. ordinal numbers). Thus, ordinary refers to the fact that the Sundays are numbered.
Ordinary Time is divided into two parts. The first and shorter part comes between the end of the Christmas Season and the beginning of Lent; the second and longer part stretches between the end of the Easter Season and the beginning of Advent.
There are usually thirty-four Sundays. There is no "First Sunday of Ordinary Time" because of the celebration of the Baptism of the Lord. The last Sunday in Ordinary Time is the solemnity of Christ the King of the Universe.
The liturgical color for Ordinary Time is green, a symbol of hope.
During Ordinary Time the cycle of the feast days of Mary and the saints are celebrated, as well as major solemnities of our Lord, such as Corpus Christi and the Sacred Heart of Jesus.
Ordinary Time does "not celebrate a specific aspect of the mystery of Christ. Rather, especially on the Sundays, they are devoted to the mystery of Christ in its fullness" (General Norms for the Liturgical Year and the Calendar, #43).
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Loyola Press offers a variety of free online resources for all the seasons of the liturgical year.
Trinity Sunday: all about Trinity Sunday in this downloadable at-a-glance resource.