History of Palm Sunday:
Beginning in the fourth century in Jerusalem, Palm Sunday was marked by a procession of the faithful carrying palm branches, representing the Jews who celebrated Christ’s entrance into Jerusalem. In the early centuries, the procession began on the Mount of the Ascension and proceeded to the Church of the Holy Cross.
As the practice spread throughout the Christian world by the ninth century, the procession would begin in each church with the blessing of palms, proceed outside the church, and then return to the church for the reading of the Passion according to the Gospel of Matthew. The faithful would continue to hold the palms during the reading of the Passion. In this way, they would recall that many of the same people who greeted Christ with shouts of joy on Palm Sunday would call for His Death on Good Friday-a powerful reminder of our own weakness and the sinfulness that causes us to reject Christ.
In different parts of the Christian world, particularly where palms were historically hard to obtain, branches of other bushes and trees were used, including olive, box elder, spruce, and various willows. Perhaps best known is the Slavic custom of using pussy willows, which are among the earliest of plants to bud out in the spring.
The faithful have traditionally decorated their houses with the palms from Palm Sunday, and, in many countries, a custom developed of weaving the palms into crosses that were placed on home altars or other places of prayer. Since the palms have been blessed, they should not simply be discarded; rather, the faithful return them to their local parish in the weeks before Lent, to be burned and used as the ashes for Ash Wednesday.