The ancient Jews had a daily calendar based on the moon. Easter is the first Sunday after the first full moon on or after the northern spring equinox. That is why it does not have a set date. Originally it matched the date of the Passover (which is celebrated on Nisan 15). But this was changed in 325AD by the council of Nicaea so now they sometimes occur at a different time.
With Christmas there are various opinions and theories as to why it is celebrated on 25th December but the date is not random.
Most of those theories are to do with the fact that ancient pagan ceremonies relating to the Roman god Saturn and the Persian god Mithra happened at that time of year, the tradition is thought to have started in Rome around 350AD. This to me demonstrates both the ahistoricity of religion and the fact that all traditions start somewhere.