Easter

Easter is

A Christian feast commemorating the Resurrection of Jesus.

2. The day on which this feast is observed, the first Sunday following the full moon that occurs on or next after the vernal equinox.

3. Eastertide.

That seems straightforward enough, aside from the difficulties of calendrical calculations. Those were so obscure that a special church conference was convened to determine which calculating system would be used to determine the date of Easter. Once that was settled, sort of (you will note that Eastern Orthodox dates are different), the feast itself is not controversial.

It gets a little less straightforward when we start looking at the etymology of the word Easter though. This much we are reasonably sure of: Easter is derived from Middle English ester, itself derived from Old English astre. There's a very clear Proto Indo-European root there; *aus-, "to shine." Derivatives of *-aus included east, Easter, and Aurora, as well as the name of the Greek dawn goddess, Eos.