Christmas Day is always on December 25th. Why are not our Lord’s death and Resurrection celebrated on the same day each year?
For the sake of convenience, the world has forsaken the Jewish calendar, which is based on the movement of the moon round the earth, in favor of the Roman calendar, based on the movement of the earth round the sun.
Now, the normal procedure of the Church is to arrange her festival days according to the accepted Roman calendar. By way of exception, however, the Church retains the Jewish calendar for the celebration of Christ’s death and Resurrection. Since the movement of the moon round the earth does not keep proportionate time with that of the earth round the sun, Easter necessarily becomes variable in relation to the Roman calendar. Easter Sunday is always the Sunday after the first full moon to occur after March 21st. It can fall on any day between March 22nd and April 25th.
The reason why the Church has retained the Jewish method in the case of the death and Resurrection of Christ is chiefly based upon the religious significance of these events. The paschal lamb of the Old Law, celebrating the liberation of the Jews from captivity in Egypt by the slaying of a lamb to preserve them from the slaughter of the children of the Egyptians, was but a type or figure of Christ, the true Lamb of God. By his death and resurrection we are liberated from the captivity of Satan. In order to bring out the identity between the figurative paschal lamb of the Old Law, and the true Lamb of God in the New, the Church insists that Easter be celebrated at that very time when the Jews used to celebrate the Passover.
In other festivals the Church follows the Roman, or rather, the Gregorian calendar, which is a modification of the Roman calendar.