Actually, it's Lady Day that coincides with Oestara.
Easter merely takes its
name from Oestara's feast. And that only in English: in the other European languages Easter is called some variant of "Pasha", and takes it's name from
pesach, or Passover. Easter's a lunar feast with its date fixed roughly according to the Christian calculation of the date for Passover; not coinciding with any of the reconstructed probably-Celtic feast-days which are all solar feasts.
Beltain coincides with the feast of St Philip and St James the Apostles with Saint James the Brother of the Lord, also known as Saint Walpurga's Day, also known as Holy Roodmas, also known as May Day.
Just for the sake of historical accuracy, the Celts did not keep written records of their beliefs or their religious practices, and existed across the entirety of northern Europe and across the better part of four millenia. So broad statements about what "the Celts" believed are approximations of historical fact, at best, and probably reflect only a fragment of what "the Celts" believed across the breadth of their culture. Also, "the Celts" did not use the Gregorian calendar until some time after Pope Gregory, so the dates of October 31 and November 1 would be meaningless to them.
What we do know about the probably-Celtic feasts that have become the basis of the neopagan reconstructed religion is that they
- were Christian feasts whose cultural importance is out of proportion to their religious importance
- Are Christian feasts that fall on the same days in different European regions, and have similar folk customs, but have different Christian names and customs
- are situated on significant dates in the solar year: solstices, equinoxes, or half-way between a solstice and an equinox
These empirical facts underpin the
theory that the days were syncretically adopted by Christians from their Celtic cultural environment. But we shouldn't lose track of the fact that what we
speculate about Celtic holidays is based on what we
know about the
Christian feast-days that occupied the solar-significant dates.