Why Is Christmas Not a Moveable Feast Like Easter?

The calculation of the date of Easter is a bit more complicated than Mr Kosloski indicates. St Bede the Venerable wrote an entire book on it, The Reckoning of Time.

From Aleteia

By Philip Kosloski

While the precise date of Easter changes each year, Christians celebrate Jesus' birth on a fixed date that never changes.

For Christians, the two biggest celebrations of the year are Christmas and Easter. Everything else revolves around those two feasts, as they recall the birth, death, and resurrection of Jesus Christ.

Yet, these two feasts are celebrated very differently. Jesus' resurrection is remembered on a different calendar day each year, as it always occurs on a Sunday and follows the Jewish calculation of Passover.

Jesus' birth, on the other hand, was fixed on the calendar to December 25. It is on a different day of the week every year, but its calendar date is not moved at all.

Why is that?

Easter's connection to Sunday

The biggest reason why Easter is celebrated on a Sunday each year is that Jesus rose from the dead on a Sunday. This detail was specifically mentioned in the Bible:

Now after the sabbath, toward the dawn of the first day of the week, Mary Magdalene and the other Mary went to see the sepulchre. (Matthew 28:1)

The Jewish people held their sabbath on Saturday and when Jesus rose on the first day of the week, Sunday, he created a "new" sabbath.

It's for this reason that Christians around the world hold their services on a Sunday. This is meant to be a constant reminder of the first Easter Sunday. In fact, every Sunday is designed to be a "little Easter."

At the same time, there is potential that Easter could be set on a fixed day of the year, as the Constitution on the Sacred Liturgy states in 1963:

The Sacred Second Vatican Ecumenical Council recognizes the importance of the wishes expressed by many concerning the assignment of the feast of Easter to a fixed Sunday and concerning an unchanging calendar. ... The sacred Council is not opposed to assigning the feast of Easter to a fixed Sunday in the Gregorian calendar, provided those whom it may concern give their assent, especially the brethren who are not in communion with the Apostolic See (e.g. Reformation Churches, Orthodox). 

In fact, for the last several years, there has been a strong push by both Catholics (including the popes) and Orthodox to unite on a common date for our celebrations of Easter. This could well happen soon.

Christmas is not connected to a day of the week

When it comes to Christmas, the Bible is silent as to the specific day of the week. We don't precisely know when Jesus was born. He could have been born on a Monday, a Wednesday or even on a Sunday. While we could try to speculate and calculate the probability of a particular day, there is no confirmation in the Bible.

The early Church then set the celebration of Christmas on December 25, on a day that many believe was the actual day of his birth.

December 25 occurs on a different day of the week every year, which has its own spiritual symbolism. Dom Prosper Guéranger comments on this in his Liturgical Year:

For firstly we may observe with the old Liturgists that the Feast of Christmas is kept by turns on each of the Days of the week, that thus its holiness may cleanse and rid them of the curse which Adam's sin had put upon them.

Furthermore, "the great mystery of the Twenty fifth of December being the Feast of our Savior's Birth, has reference not to the division of time marked out by God himself and which is called, the Week, but to the course of that great Luminary, which gives life to the world because it gives it light and warmth."

Easter and Christmas remain the firm pillars of the liturgical year and even though they are celebrated differently, they both have specific symbolism that helps us remember them in a unique way.