Palm Sunday is a Christian moveable feast that falls on the Sunday before Easter. The feast commemorates Jesus' triumphal entry into Jerusalem, an event mentioned in each of the four canonical Gospels.

In Catholic, Episcopal, and other denominations, and in the Ethiopian Orthodox Church, worship services on Palm Sunday include a procession of the faithful carrying palms, representing the palm branches the crowd scattered in front of Jesus as he rode into Jerusalem. The difficulty of procuring palms in unfavorable climates led to their substitution with branches of native trees, including box, olive, willow, and yew. The Sunday was often named after these substitute trees, as in Yew Sunday, or by the general term Branch Sunday.

From Wikipedia