Taken from Living Liturgy 2012:
These thirty-four weeks of non-festal time are named "Sundays in Ordinary Time," first, because they derive non from a feast day as such (they use to be called Sundays "after Epiphany" or "after Pentecost") but from the mystery of Jesus Christ. These Sundays, then rather than celebratings a particular saving event (such as Christmas, Easter, pentecost), actually celebrate the whole mystery of Christ. For this reason, every Sunday might also be called a "cominical" feast day, that is, a day celebrating the Lord (from the Latin Dominus, Lord).
Second, they are called "Ordinary Time" because they are numbered or "counted time" (from the Latin word ordinarius and its cognates, which mean "according to order," "regular"). Although the Latin is translated from the editio typica (the official, Latin version of our liturgical texts) in English by "ordinary," we must not think these Sundays are prosaic or unimportant. In fact, they make up the longest liturgical season of the year, the church's teaching time. This is the time when, Sunday after Sunday, we walk with Jesus through a gospel and learn what it means to be his followers, what it means to hear and heed his call; and we learn how to come to deeper relationship with him.
