The thing is, Easter can be as early as March 22 or as late as April 25. So it really is dependent on being that flexible.
The date of Easter Sunday is usually the first Sunday after the first Full Moon occurring on or after the March equinox.
Plus Easter is the kind of Holiday that is a little too specific for a setting for most religions have been replaced with the Church of the One. Christmas, while more commercialised, is still about charitable endeavours so you could have an event based on doing some good. Likewise, Halloween isn't tied to a particular religion, its just about scaring people.