During a burial today, I noticed the coffin was buried facing east.
Where does this Christian tradition come from?
It's not a purely Christian tradition. Other religions practice this as well. There's an article here on ehow.com that gives an overview of the origins of the practice. Some highlights:
According to "Ethnicity and the American Cemetery," the feet of the deceased face east as well. This tradition is based on the belief that when Jesus returns, the departed will rise from the grave already facing his direction.
According to Northumberland County Council, the tradition began when Pagans buried the dead so they would face the rising sun.
Also note that it's not universal. We have a cemetery tied to a Church a few miles from here where the graves all point to the North, and several others I've visited have some facing one way, and others at right angles - those graves are laid out to maximize use of the land.
Most churches are built with the altar at the east end, expecting the Second Coming of Christ to be from the East: New International Version of Matthew 24:27 - "For as lightning that comes from the east is visible even in the west, so will be the coming of the Son of Man." Christian graves also tend to face east, but the geography of the cemetery can be more influential.
In the Judeo-Christian tradition the East was the direction of prayer since the time of Abraham. Genesis mentions at least once, that he got up in the early morning facing East toward the rising sun to pray (The Sun being an early symbol of God benevolence). As others have noted this has also been the tradition when it comes to building sacred structures like churches, synagogues since the time of the Tabernacle in the Old Testament onward and in orientating their altars for Jews, Catholics, Orthodox Christians and a some High Church Protestants. So it natural that this sort of thing would be carried over to cemeteries since those places are considered sacred spaces and facing east is an ancient liturgical custom for having reverence for God.
At one time, it was true that Catholics must be buried facing the east. It is no longer true, and it was part of the same Church Law which said that the body of the faithful departed should face the altar in a Catholic Church thus facing the east. During the Funeral Mass of a priest, the corpse is reversed as to be facing his congregation.
As anywhere, you may find older Catholic cemeteries that still accommodate this tradition.
There is no requirement in church law regarding the position of burial. The only thing that comes close is a reference in the Roman Ritual of 1950 (no longer in effect) that stated that at the funeral Mass the coffin was to be placed so that the feet of the deceased should point toward the main altar.
Presuming the church had been constructed so that a priest celebrating Mass would face the east, the deceased at the funeral would face in that same direction.
Although there was no canonical requirement, it was in fact customary for people to be buried facing the east, reflecting the traditional Christian belief that, at the end time, Jesus would return from the east. (An even earlier pagan tradition had the deceased buried facing the east because it was where the sun rose.)
Correctly, you suggest that the same values guided the traditional placing of a church’s main altar, so that the priest celebrating Mass could face the east. This evoked the Christian expectation as to Christ’s return. (See Mt 24:27: “For just as lightning comes from the east and is seen as far as the west, so will the coming of the Son of Man be.”) - Must one be buried facing east?
There is no fixed rule nowadays and I know of some very traditional Catholic Benedictine cemeteries where the brethren are buried facing the south due to limited burial space.
Some people believe that God moves east to west. At the second coming of Christ at the end of the tribulation He will come in from the East moving to the west. The idea was too bury the bodies facing east so that when the rapture happens they will already be facing toward His coming. This is more tradition than it is biblical but they did it for that reason.
On the other hand as a Jew you always approached God as at the tabernacle moving east to west. This was important as the pagans always worshiped facing east so in a way you should bury them facing west as not to face the pagan rising sun. How a Jew in the Old Testament or a Christian in the church age and how they are buried is not important..