Unique among western holidays, Easter's date is determined by the sun and moon as well as the calendar: it is the first Sunday after the first full moon following the spring equinox. This year, the full moon comes the day after the spring equinox, so Easter is early; if the equinox falls just after a full moon, Easter can be as late as April 25. The Orthodox Church uses a different method of calculating the date of the full moon than western churches, so the Orthodox Easter is often on a later Sunday than the western Easter.
That's not the only confusion surrounding Easter: the origins of the holiday get confused too. Easter was originally called Pasch and was celebrated immediately after Passover, since Christ was crucified on the eve of Passover and resurrected after it; early Jewish converts to Christianity saw the new feast as an extension of Passover. When the official Christian calculation of the date was set in 325, it was set on the Sunday following the full moon, to avoid having Easter celebrated during Passover.
As Christianity spread, it co-opted many pagan rituals; in particular, the Christian celebration of resurrection fit neatly with pagan fertility rituals that tended to be celebrated around the spring equinox. The name "Easter" appears to come from the Germanic moon goddess, Eostre, whose sacred day was the first full moon after the equinox; she was accompanied by a sacred rabbit and carried a basket of eggs. The Church neatly absorbed the accouterments and name when it overlaid the fertility rituals with Christian rituals, giving us our Easter bunny and Easter eggs.
So we have a Christian holiday tied to a Jewish feast and named for a pagan goddess, celebrated on a different date each year... I think I'll just go hide Easter eggs.