Well the Crucifixion is specifically given as taking place on a Friday in the Gospel accounts. And it's set around this time of year, because it's Passover time.

It's not an anniversary thing, it's a lot more complicated. The Eastern Christians and the Western use a different system for deciding when Easter should be, and most times these don't come up with the same date.

For one reason or another the system for calculating Easter and Passover are also different, so more often than not they don't coincide, though they really should.

Christmas works on a different basis, and there's no attempt to tie up the dates for Christmas and Easter. Why should there be? People aren't celebrating a date, they're using a date to celebrate an events.