The 40th day of Easter is Ascension Thursday. On this day, the Lord Jesus ascended into heaven from the Mount of Olives. As a cloud took Him from the sight of the disciples, "suddenly two men dressed in white garments stood before them. They said, 'Men of Galilee, why are you standing there looking at the sky? This Jesus who has been taken from you into heaven will return in the same way as you have seen Him going into heaven."
The Lord Jesus will return in the same way He was seen going into heaven. "As you have seen Him going into heaven": The Son of Man will come back, descending upon the clouds - this is what the Lord Jesus said about the parousia. Also, there is a long standing tradition that the Mount of Olives will be the place of the Lord's return. He ascended from the Mount of Olives, He will return on the Mount of Olives.
This is one of the explanations given for the Christian Liturgical Orientation. Inasmuch as the Mount of Olives is located east of Jerusalem, Christians faced the east when they prayed, as Jews faced Jerusalem when the prayed. Between Mount Zion (where the temple of Jerusalem stood) and the Mount of Olives (where Jesus ascended into heaven) is the Valley of Jehosaphat which is the known as the valley of judgment. Jews believe that from that valley God will judge all nations. Thus, on the slopes of the Mount of Olives, there is a Jewish cemetery where the dead are buried with their faces towards Jerusalem. This they do so that at the resurrection of the dead, those buried there will rise facing the Temple. However, on the slopes of Mount Zion, there is a Christian cemetery where the dead are buried with their faces towards the Mount of Olives. This they do so that at the resurrection of the dead, those buried there will rise facing the Lord who will descend on that mountain. Usually, we Christians bury our dead with their faces towards the east in anticipation of the Parousia.
"When we eat this bread and drink this cup, we proclaim the death of the Lord until He comes in glory." The celebration of the Liturgy does not only look back at the Paschal Mystery which is made present. It also looks back with blessed hope towards the return of our Lord Jesus. Thus, we face the east - if not the literal east, at least the "spiritual east" which is the Cross. In doing so, we engage in a procession towards the Lord "who shall come to judge the living and the dead."