The answer
The Coptic Orthodox Church uses the Julian calendar while most of the churches in the west use the Gregorian calendar.Fr. John Ramzy wrote two very nice articles worth reading:
1) The Glorious Feast of Nativity: 7 January? 29 Kiahk? 25 December?
.http://www.suscopts.org/resources/literature/148/the-glorious-feast-of-nativity-7-january-29-kiahk-/
2) The Date of the Glorious Feast of the Resurrection
.http://www.suscopts.org/resources/literature/159/the-date-of-the-glorious-feast-of-the-resurrection/
Answered by His Eminence Metropolitan Youssef (opens in a new tab)