Again, thanks to George Grant...
"Easter is early this year. Very early. Unusually early.
"The "Great Pascha" is always the first Sunday after the first full moon after the Spring Equinox. This is based on the lunar calculations of the Hebrew calendar--because, of course, it is connected to the Passover festival. That is why the observance of the holiday moves around on our Western solar calendar.
"So, while it is actually possible for Easter to be a day earlier than it will be this year, that is more than a little rare. In fact, this year’s celebration of Christ’s resurrection is the earliest Easter any of us will ever see for the rest of our lives. Indeed, only the most elderly members of our churches have ever seen it this early before--they'd have to be 95 or older because the last time Easter came so soon was in the year 1913. And the next time it will be this early will be more than two centuries hence, in the year 2228."