The Collaborative International Dictionary
Palm Sunday \Palm" Sun`day\ (Eccl.) The Sunday next before Easter; -- so called in commemoration of the triumphal entry of Jesus Christ into Jerusalem, when the multitude strewed palm branches in the way. The event is commemorated in Christian churches by distribution of blessed palm leaves.
Wikipedia
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 many Christian denominations, 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.
Palm Sunday is a 1981 collection of short stories, speeches, essays, letters, and other previously unpublished works by author Kurt Vonnegut Jr.