Originally Easter celebrations were time based on the Jewish calendar, since the Last Supper is connected to Passover (though there’s debate about whether or not it was a seder).
However there was a lot of argument in antiquity about how to run a calendar correctly; Greek astronomers in particular had a tough time agreeing on a calendar system that kept the solar year in line with the seasons but also respected the lunar months which most people actually used (the seven-day week didn’t become common until fairly late).
Around the 4th century, as Christianity spread widely, lots of Christians didn’t have a local Jewish community whose Passover observations they could imitate. Local churches all over cobbled together their own methods of calculating Easter (in Latin this became known as the computus). Eventually they settled on a method which used the Roman year (the Julian calendar, originally put in place by Caesar) as the basis for the calculation.
In 1582 the Catholic church adopted the Gregorian calendar, which does a better job of tracking the solar year. The Orthodox churches, though, still use the Julian Calendar for religious observances, which is why the dates of the celebration differ.
The Cultural Easton 