Because they're using a different calendar.
Violent clashes broke out this week between Coptic Christians and Egyptian security forces, following a New Year's Day suicide bombing at a church in Alexandria that killed 21 people. Copts blame authorities for not taking the escalating violence against Egyptian Christians seriously. Authorities are now on high alert in anticipation of more violence on Jan. 7, when Copts celebrate Christmas. But why do the Copts celebrate the Christmas holiday on a different day from Western Christians?
Because they're still using the Julian calendar. Like the Eastern Orthodox Church in Russia, the Copts still use the Julian calendar adopted by Julius Caesar in 45 B.C. The Julian calendar has 365 days, with a leap year added every four years, but each year is about 11 minutes too long, meaning that over time it has come to be out of sync with the more accurate modern Gregorian calendar, named for Pope Gregory XIII. So though Egypt, including its Coptic citizens, currently uses the Gregorian calendar for most other affairs, Christmas will still be celebrated on Jan. 7 -- for at least the next few decades.
Although they agree on dates, there are crucial differences between the Coptic Church and Eastern Orthodox traditions. Egyptian Christianity dates back to the founding of the Church of Alexandria by St. Mark in 43 A.D., making it one of the world's oldest continuous Christian denominations. The Copts' distinct identity comes from their rejection of the Council of Chalcedon in 451, which put forth the notion that Christ had separate divine and human natures. Those who rejected the concept, believing that Christ had one unified nature, were referred to as "Monophysites," though modern Copts reject the term as insulting and inaccurate. Those who accepted the council were called Dyophysites. The descendants of the anti-Chalcedonians are today referred to as the Oriental Orthodox Church and include the Copts as well as the Ethiopian, Syriac, and Armenian churches and a few others. What's today referred to as the Eastern Orthodox Church -- which includes the Russian and Greek churches and is distinct from the Oriental Orthodox Church despite the similar names -- split off from Western Catholicism later.
In recent decades, relations between the different branches of Orthodoxy have improved, including agreements on the recognition of joint marriages and baptisms. Some church leaders hope the Orthodox branches of Christianity will eventually unify into a common hierarchy, though that's still a long way away. The Coptic Church maintains its own distinct clerical heirarchy, currently led by Pope Shenouda III.
While the Coptic Church's relations with other Christian denominations have substantially improved, Christian-Muslim relations in Egypt, where Christians represent about 10 percent of the population, are at a low point. Six Copts and a Muslim security guard were killed in a shooting at a church in Cairo last Coptic Christmas. Two Christians were later killed in protests over a church permit near Cairo. The comments of some Coptic leaders, such as one bishop who suggested recently that verses were added to the Quran after the Prophet Mohammed's death, have also inflamed tensions.
The violence is reminiscent of the frequent attacks against Copts in the late 1990s, though using bombs against churches is a new tactic. No group has yet taken credit for the most recent attack, though an al Qaeda-linked group threatened the Egyptian Copts after an attack on Iraqi Christians in November.
Thanks to His Grace, David, General Bishop of the Coptic Archdiocese of North America.