The date of Easter isn't calculated from the phases of the real Moon. Instead, a set of complicated rules were devised by an astronomer named Christopher Clavius at the request of Pope Gregory XIII in the late 16th century, as part of the calendar reform which gave us the Gregorian calendar that we use today.
Use the Easter Calculator to see how the date of Easter is calculated using the rules which Clavius devised.
Copyright 1995-99 by David Harper and Lynne Marie Stockman