Why is Easter a “movable” feast? I know it needs to be on a Sunday, but how does the Church decide which Sunday?
There are three key elements to the Easter story that form the basis for Easter taking place as it does each year – each based upon the calendar of Israel. The first is that of the Resurrection of Jesus having happened “early in the morning on the first day of the week.” While we might consider Monday as the first day, in Israel the Sabbath is understood as the 7th day (Saturday as we know it). Thus, the day of the week when Jesus was raised up was on Sunday.
Second, the death and resurrection of Jesus took place during the Passover. The beginning of Passover can take place each year on a different day of the week (as it is based on the full moon during the month of Nisan [that is, on the 14th day of Nisan] in the Jewish Calendar). In the year of Jesus’ crucifixion, death, and resurrection, this date fell on either a Wednesday or Thursday (it is debated, even within Scriptural references). Though this date can fall on any day of the week, it is significant in connection with Christ’s three days from death to the resurrection that it fell in that given year such that Sunday would be the third day – which as we know is of greatest significance to we Christians.
Finally, the third element is the vernal equinox – better known to us as the first day of spring. Since the 4th Century, the Church has always placed the dating of Easter after this date. Thus, we celebrate Easter on the first Sunday after the first full moon (Nisan 14) after the first day of Spring. This is why Easter can be anytime between March 22 and April 25, as all three of these elements, in their given order here, are determining factors.
In the [liturgical calendar used for the Extraordinary Form Mass of the 1962 Missale Romanum], it seems like the Gloria and Alleluia are displaced before Lent begins. What is the significance of this?
In the liturgical calendar for use with the Extraordinary Form Mass, there are a series of weeks that lead up to the 40 days of Lent that are preparatory for the more intense time of repentance and discipline within Lent. These weeks include three Sundays, whose names are reminiscent of 70 days (Septuagesima Sunday), 60 days (Sexagesima Sunday) and 50 days (Quinquagesima Sunday) before Easter. (For what it is worth, the word for “Lent” in Latin is Quadragesima – which indicates the number “40”).
The “burying” of the Alleluia on Septuagesima is significant as a “warning” of the coming of Lent (and of death). Many of the elements of the pre-lenten Sundays are similar to those of Lent (including as mentioned in the question the disuse of the Gloria, along with the wearing of violet vestments). During these weeks, fasting and penances may be embraced toward preparing for the required time of fasting that will soon arrive (on Ash Wednesday). Thus, the people are forewarned by these symbolic elements, along with a change in the tone of some of the texts of the Mass, which begin to speak more of penance and preparing for death while asking for the grace of salvation. Likewise, all are invited to already begin penitential acts toward a more fruitful way of penance during Lent itself.