Why is the date of easter determined by the equinox and the full moon afterwards? All other commonly celebrated days in western civilization seem to be on fixed dates.
Unless you’re Jewish.
Perhaps this will answer your question: Date of Easter FAQ from the US Naval Observatory. (They’re the offical timekeepers for the U.S. government, which is why they know this stuff.)
I dated an Easter once.
Are you egging us on with that, or waiting for the rabbit to die?
The Naval Observatory FAQ did a good job of answering how the date is calculated, but it did not really address why.
The Catholic Encyclopedia - Easter Controversy gives a fairly thorough history of the decisions.
The (extremely) short answer is that the Jewish Passover (which is the marker date for the events of Holy Week) does not occur on a fixed date in the solar calendar or on a fixed day of the week. The early Christians first decided to set the celebration to always occur on Sunday, then decided that (while they wanted it near Passover for historical legacy), they rejected using the Jewish calculations out of spite. Having determined those two guidelines, they then chose to place it as near to a full moon as possible so that pilgrims would have more light to travel by.