This year, we celebrated a Vesperal Liturgy on Christmas Eve and an Orthros and Divine Liturgy on Christmas Day. I can't recall the term our bishop uses to allow (reluctantly it seems) this practice.
It's only been in recent years that we celebrated a Liturgy on Christmas Day also. Previously, an Orthros (9:30p.m.) and Divine Liturgy was celebrated at 10:30p.m. It was later moved back to 7:00 p.m. Orthros, Divine Liturgy at 8:00 p.m.
When the Christmas Day Divine Liturgy was added, the Christmas Eve Vesperal Liturgy was later moved back to 7:00p.m., and last year, to 6:00 p.m.
These time changes were intended to see if attendance would be increased. Attendance was always good, but not excessive as it had been in our early days (early 1970's).
This year, we began the Vesperal Liturgy at 5:00p.m. and had record breaking attendance, standing room only, and our church is large, especially compared to other Orthodox Churches in the U.S. The Christmas Day Liturgy, (Orthros at 8:15 a.m., Liturgy at 9:30 a.m.) attendance was a little higher than usual, but that may be attributable to the fact that it was Sunday (a few who attended the previous evening's Vesperal Liturgy attend the Christmas Day Liturgy too.)