The Liturgical Calendar
In the western church, the liturgical year begins with Advent, which consists of the four Sundays before December 25. The readings for these days are designated Advent 1 through Advent 4.
Special readings for Christmas begin on December 22 and continue to January 2. These days are designated by their dates. January 1 is an exception. It is designated as New Year's Day. Since it is also designated as the Feast of the Holy Name for Lutherans, Catholics, Anglicans, and Episcopalians in the United States, those readings are also provided on that day. (Episcopalians in Canada and perhaps others celebrate it on August 7.)
Epiphany is celebrated on January 6. The readings begin on January 3, and the weeks that follow are designated as Epiphany 2 through 9. The actual number of weeks in Epiphany depends on the date of Easter. Some churches designate the time after Epiphany and before Lent as Ordinary Time. These readings designate the Sundays as being in Epiphany. Although the designation differs, the readings are identical.
The season of Lent follows Epiphany. Lent begins 40 days before Easter, not counting Sundays. The weeks are designated Lent 1 through 6.
The days beginning the Monday before Easter are designated as Holy Week. The following weeks are designated as Easter 2 through 7.
Pentecost follows Easter. The readings are only for that week.
Trinity follows Pentecost, and the readings are only for that week.
The readings following Trinity are designated as Ordinary Time. The number of the first week depends on the date of Easter. (Pentecost is eight weeks after Easter, and Trinity is one week after Pentecost, so Ordinary Time begins ten weeks after Easter, and thus may be as early as May 24 or as late as July 4.)
Some denominations designate this period as the Season after Pentecost, and number the weeks differently. Although the designations differ, the readings are the same.