Calhoun NAILED IT

.
The back story that I heard/read was:
Roman Emperor Constantine was the first emperor to embrace Christianity. However, he was bothered by many Christians taking different days off from work to pilgramage to Calvary. The AFL/CIO had not yet formed, so Constantine was in charge and asked the Christian leaders to meet and decide on a fixed day. The leaders met ,argued, but couldn't agree (sorta' like our congress). So Consty stepped in and issued the following decree (sorta' like an excitive order):
(1) Easter should be on a Sunday as that was the Christian day of worship.
(2) The Resurrection is symbolic of re-birth. Spring is also symbolic of re-birth. Easter should be after the turn of Spring.
(3) The mode of travel of most Romans on their Pilgramage was walking day and night. There was very few street lights back in Rome then. Walking under a full moon would be easier.
There you have it and no Elvis, I wasn't in town yet back in 300 AD

. Like our rules book there has to be an exception:
Full moons differ, depending on where on our planet you call home. In case the moon is full in part of the planet but not the other when Spring comes, the tie breaker is if full over Greewich meridian after Spring equinox.