Besides the times of the year that have their own distinctive character, there remains in the yearly cycle thirty-three or thirty-four weeks in which no particular aspect of the mystery of Christ is celebrated, but rather the mystery of Christ itself is honored in its fullness—the Church contemplates the entire mystery of salvation—especially on Sundays. This period is known as Ordinary Time.
But “Ordinary” does not mean boring or ho-hum, but the word “ordinary” comes from the word “ordinal” which means counted (i.e. First Week of Ordinary Time, Second Sunday of Ordinary Time, etc.).
Ordinary Time then, is a period that is counted that doesn’t fall in Lent, Easter, Advent, etc. Ordinary Time begins at the conclusion of the Feast of the Baptism of the Lord, which is also the end of the Christmas Season.
Ordinary Time has two defined periods: the first period begins after the Baptism of the Lord and continues through the day before Ash Wednesday; and the second begins after Pentecost and continues to Evening Prayer on the Saturday before the First Sunday of Advent. (10:8)