The Feast of Easter as Described by the The Catholic Encyclopedia (1917).

Easter is the principal feast of the ecclesiastical year. Leo I (Sermo xlvii in Exodum) calls it the greatest feast (festum festorum), and says that Christmas is celebrated only in preparation for Easter. It is the centre of the greater part of the ecclesiastical year. The order of Sundays from Septuagesima to the last Sunday after Pentecost, the feast of the Ascension, Pentecost, Corpus Christi, and all other movable feasts, from that of the Prayer of Jesus in the Garden (Tuesday after Septuagesima) to the feast of the Sacred Heart (Friday after the octave of Corpus Christi), depend upon the Easter date.... The connection between the Jewish Passover and the Christian feast of Easter is real and ideal. Real, since Christ died on the first Jewish Easter Day; ideal, like the relation between type and reality, because Christ's death and Resurrection had its figures and types in the Old Law, particularly in the paschal lamb, which was eaten towards evening of the 14th of Nisan….The connection between the Jewish and the Christian Pasch explains the movable character of this feast. Easter has no fixed date, like Christmas, because the 15th of Nisan of the Semitic calendar was shifting from date to date on the Julian calendar.

The First Council of Nicaea (325) decreed that the Roman practice should be observed throughout the Church. But even at Rome the Easter term was changed repeatedly. Those who continued to keep Easter with the Jews were called Quartodecimans (14 Nisan) and were excluded from the Church. The computus paschalis, the method of determining the date of Easter and the dependent feasts, was of old considered so important that Durandus (Rit. div. off., 8, c.i.) declares a priest unworthy of the name who does not know the computus paschalis. The movable character of Easter (22 March to 25 April) gives rise to inconveniences, especially in modern times. For decades scientists and other people have worked in vain for a simplification of the computus, assigning Easter to the first Sunday in April or to the Sunday nearest the 7th of April. Some even wish to put every Sunday to a certain date of the month, e.g. beginning with New Year's always on a Sunday, etc. [See L. Günther, "Zeitschrift Weltall" (1903); Sandhage and P. Dueren in "Pastor bonus" (Trier, 1906); C. Tondini, "L'Italia e la questione del Calendario" (Florence, 1905).]

From “The Catholic Encyclopedia (1917).