Perhaps I can help here. I noticed this thread after answering Martin's question about Passover in another thread.

The original name for the Friday of Holy Week, as Susan has indicated, was Great and Holy Friday. In the Eastern Church, we still call it that. The Crucifiction happened following the last Seder of Passover. The Gospels talk about the need to make sure Jesus was dead and his body removed before sundown because, according to law, they couldn't be crucifying anybody on the Sabbath.

The celebration of the Jewish Passover, as the whole Jewish calendar, is based upon phases of the moon rather than the earth's path around the sun. The Paschal (Passover) full moon is the first full moon following the vernal equinox. The Christian Feast of the Resurrection, then, is the first Sunday following the Paschal full moon. At some point an additional provision was made in Canon law so that Pascha may not coincide exactly with the Jewish Passover but must follow it. Also in Canon law, March 21 was set as the constant date for the vernal equinox. Astronomers know that this date isn't constant but that calculation was beyond the ability of the early Church. They were looking for a regularly repeating system to use in designating the dates for the major moveable feasts.

The reason the date for Pascha is often different for Eastern and Western churches is that the Eastern Church still uses the Julian calendar (the state calendar in use at the time the Church began) for reckoning the dates for moveable feasts. So March 21 (the date designated as the vernal equinox) falls 13 days later for the Eastern Church than for those in the Western tradition. So it's possible, as in this year, to have Eastern and Western Pascha fall on the same date but more often, Eastern Pascha is later.

The date for Christmas is a little different story. The ancient Christian Church didn't have a winter solstice celebration so the Church turned the the Nativity of Christ into a much bigger deal and moved the Feast Day to December 25. The idea was to coopt a big pagan celebration that they couldn't get people to quit having. The Eastern Church continued to celebrate Christmas using the old Julian calendar until early in the 20th century. Eventually, all but the most intransigent traditionalists adopted the New (identical to but not called Gregorian) calendar for all the fixed Feast Days. Some Eastern groups still use the Julian calendar for even the fixed Feasts so, for them, Christmas currently falls on January 7 but on their religious calendar, it's still December 25.

I'm not sure how much I added here but at least it's all in one place.

      - Mark