According 2 https://en.wikipedia.org/wiki/Palm_Sunday Palm Sunday is a Christian moveable feast https://en.wikipedia.org/wiki/Moveable_feast_(observance_practice) that falls on the Sunday before Easter. https://en.wikipedia.org/wiki/Easter The feast commemorates Jesus’ triumphal entry into Jerusalem. https://en.wikipedia.org/wiki/Triumphal_entry_into_Jerusaleman event mentioned in each of the four canonical Gospels https://en.wikipedia.org/wiki/Gospel#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. https://en.wikipedia.org/wiki/Jerusalem https://en.m.wikipedia.org/wiki/Palm_branch_(symbol)The difficulty of procuring palms in unfavorable climates led to their substitution with branches of native trees, including box, https://en.wikipedia.org/wiki/Buxus_sempervirens olive, https://en.m.wikipedia.org/wiki/Olive willow, https://en.m.wikipedia.org/wiki/Willow and yew. https://en.m.wikipedia.org/wiki/Taxus The Sunday was often named after these substitute trees, as in Yew Sunday, or by the general term Branch Sunday.