Why Do Some Orthodox Churches Use Different Calendars?

Orthodox Christians do not all use the same calendar for fixed feast days. Some Churches continue to use the Julian calendar, while others use the Revised Julian calendar for fixed feasts.

Does this change the faith itself? No. The calendar determines when a feast is observed; it does not change what the Church believes about Christ, the Theotokos, the saints, or the feast being celebrated.

Why can Christmas fall on different civil dates? December 25 on the Julian calendar currently corresponds to January 7 on the civil Gregorian calendar. Churches using a calendar aligned with the civil calendar celebrate December 25 on December 25 civil date.

What about Pascha? The calculation of Pascha follows a separate ecclesiastical system and should not be reduced simply to the fixed-calendar question.

For the Church year as a whole, see What Are the Twelve Great Feasts?.