Yeah, I think they kept it a Friday because according to the Bible, Jesus was crucified on Friday, the Sabbath was the next day, and then he was resurrected on Sunday. So keeping the celebrations on those days fits in well with the normal church calendar, that you would celebrate the resurrection on Sunday. Eta: I can't remember if the passover seder was supposed to be on the sabbath or the last supper, hang on and I'll try to look it up. Sorry, I didn't grow up high church; the liturgical calendar isn't really our thing. Edit 2: OK, so Passover was the last supper, the rest of the Passover festival continued for the next several days. It was a bit confusing, but apparently there was confusion over calendars because of leap days and such, so the church decided to follow the lunar schedule they follow now to try to keep things relatively simple, celebrate the day in approximately the right time (nothing magical about the specific date, just a day of remembrance), and have Easter on Sunday, as the first Resurrection Sunday.