In truth, every Sunday is a "Proper". The term Proper just refers to the part of the service that changes Sunday to Sunday based on the Season. Most of the Seasons in the Church Calendar, however, last a specific amount of time (ex. Lent last 40 days plus 6 Sundays) or even have a particular starting point (ex. Advent starts 4 Sundays before Christmas Day on December 25).
The Season after Pentecost, though, varies in length and in starting point. It ends when Advent starts, but it begins after the Day of Pentecost. Pentecost occurs 50 days after Easter, and Easter's date varies every year based on the Hebrew Calendar for the Passover. Thus the number of Sundays after Pentecost varies every year.
The Propers help to designate what we read based on the 3 year Lectionary cycle. Each Proper in the Season after Pentecost falls on the Sunday closest to a specific date (ex. Proper 23 falls on the Sunday closest to October 12). While the readings for the 18th Sunday after Pentecost in Year C of the Lectionary, for example, will vary, the readings for Proper 23 in Year C always stay the same.
This is also why I title my sermons the way I do. For the Season after Pentecost, I always include the Proper in the title so you will know what readings were used for that particular sermon.
Sources Used:
- Cross, F.L. and E.A. Livingstone eds. The Oxford Dictionary of the Christian Church. 3rd edition revised. New York: Oxford University Press, 2005.