That's a great question! The Armenian Apostolic Church celebrates Christmas on January 6th instead of December 25th because they follow the Julian calendar. The Julian calendar is an ancient calendar that is 13 days behind the Gregorian calendar. This means that the Armenian Christmas falls on January 6th, which is actually December 25th on the Gregorian calendar. So, while most Christians celebrate Christmas on December 25th, Armenians celebrate it a little bit later.