Ordinary Time gets its name from the word ordinal, meaning “numbered,” since the Sundays of Ordinary Time are expressed numerically. Ordinary Time occurs outside of other liturgical time periods. Essentially then Ordinary Time is that part of the year that does not fall within the seasons of Advent, Christmas, Lent, or Easter. The Catholic Church celebrates two periods of the year as Ordinary Time. In the United States, the first period begins after the Masses have been said on the evening of the Feast of the Baptism of The Lord (the Sunday after The Epiphany), meaning that the feast itself falls within Christmastide, but the whole day does not. The next Sunday is still reckoned “The Second Sunday in Ordinary Time,” because it is the Sunday of the second week in Ordinary Time.
It was not until after Vatican II that the term “Ordinary Time” was officially used to designate the period between Epiphany and Lent, and the period between Pentecost and Advent. Rather than being called the “Season of Ordinary Time,” the times were called “Season After Epiphany” and “Season After Pentecost” After the new Catholic Calendar took effect in 1969, these older designations were no longer used in the modern Roman calendar. However, these former designations are retained in the Extraordinary Form of the Roman Rite.