Ordinary Time is a term used in the Christian (especially the Roman Catholic) liturgical calendar to refer, collectively, to two different seasons of the liturgical year. This is the time in which the Sundays are not immediately related to either Christmas or Easter.
The first of the two seasons usually begins on the Monday following the first Sunday after Epiphany, which is observed on January 6, except that Roman Catholics in the United States celebrate this feast on the Sunday after the first Saturday in January. On the Sunday after January 6, the Feast of the Baptism of the Lord is observed in place of the first Sunday in Ordinary Time (if this falls on January 7 or 8, American Catholics move the Baptism feast to the following day as the Epiphany would occur on the Sunday), but the ensuing week is reckoned as the first week within Ordinary Time, which continues until Ash Wednesday, the beginning of Lent. Thus the first section of Ordinary Time may consist of anywhere from four to nine weeks, depending upon how early or late Easter falls in a given year. It could end as early as February 3 or as late as March 9.
The second section of Ordinary Time begins on the day after Pentecost and continues until the first Sunday of Advent five or six months later. However, the weeks are numbered in such a manner as to ensure that the last Sunday before Advent (celebrated as the Solemnity of Christ The King) always takes the place of the 34th Sunday in Ordinary Time, even though, in most years, the sum of the number of weeks of Ordinary Time before Lent and the number of weeks between Pentecost and Advent is actually 33 and not 34 (specifically, all years which begin on a Sunday or Monday and leap years which begin on a Saturday will have 34 such weeks while all other years contain 33, meaning that the liturgical year will have 33 ordinary weeks approximately 68 per cent of the time). This factor, combined with the fact that Pentecost, Trinity Sunday and, in the Roman Catholic Church in the United States, the feast of Corpus Christi, preempt numbered Sundays in the series, means that at least three, and more commonly four, numbered Sundays end up being omitted each year.
In addition, certain solemnities and feasts that fall during Ordinary Time will preempt numbered Sundays in the series when the observance in question falls on a Sunday; these include, in the Roman Catholic calendar, any day that is a holy day of obligation, along with certain other special days, such as the Presentation of the Lord (formerly known as Candlemas, February 2), the birth of John the Baptist (June 24), the Solemnity of St. Peter and Saint Paul (June 29), the Transfiguration (August 6), the Triumph Of The Cross (formerly known as Holy Cross Day, September 14), All Souls Day (November 2), and the Dedication of (the basilica of) St. John Lateran (November 9).
Green vestments are used during both of the periods that comprise Ordinary Time.
The term Ordinary Time was invented at the Second Vatican Council, and the new liturgical calendar including it took effect on the first Sunday of Advent in 1969 (before this the two seasons making up Ordinary Time were known as the season after Epiphany and the season after Pentecost respectively).