Orthodox Christians use a different calendar than Western Christians. They use the Julian calendar, named after Julius Caesar, that was used for hundreds of years. The problem was that it was not exact and eventually was off by a couple of weeks.
   
Pope Gregory XIII promulgated a more precise calendar in the 16th century, which is the one that we use. Most of the world uses the Gregorian calendar, but the Orthodox Churches still use the Julian calendar for liturgical purposes.
   
Orthodox Christians celebrate Christmas this Tuesday, Jan. 7, though that day is Dec. 25 on the Gregorian calendar.

While the Gregorian calendar serves to count time so that we can all work together, the Church's liturgical calendar serves as a meditation on the life of Christ, repeating each year. 
    
The Church year begins with the First Sunday of Advent, followed by the Advent and Christmas seasons, and concludes with the Feast of Christ the King. Within the year, we will celebrate Lent and Easter, focusing on the Passion and Resurrection of Christ.
    
We are in Year C of the three-year cycle of Gospel readings at Sunday Mass, so we will be reading from the Gospel of Luke.