Easter occurs on different dates each year because, like the Jewish Passover, it is based upon the vernal equinox, that dramatic moment when the hours of the day-light and the hours of darkness at last draw parallel and then the light finally and triumphantly wins out. Thus Easter is always fixed as the first Sunday after the first full moon following the spring equinox. It's a cosmic, solar, and lunar event as deeply rooted in religious traditions originating from sun-god worship as one could conceivably imagine.