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 all four canonical Gospels.

In many Christian churches, Palm Sunday includes a procession of the assembled worshipers 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, yew, willow, and olive. The Sunday was often designated by the names of these trees, as in Yew Sunday, or by the general termBranch Sunday.

Source: catholic.org (See this site for much more information on how other countries and cultures celebrate Palm Sunday.)