On the Sunday before Pascha the Church keeps Christ's entry into Jerusalem, when the crowds spread their garments and palm branches in His path and cried “Hosanna to the Son of David.” It is a coronation and a paradox: the King comes, but riding on a humble colt, and He enters the city not to seize a throne but to be lifted up on the Cross.
The feast is the threshold of Holy Week. The same crowds that greet Him will, within days, call for His death; and the Church, holding palms, stands knowingly between the acclamation and the Passion. To keep Palm Sunday is to walk into the week of the Lord's suffering with our eyes open, singing His praise even as we follow Him toward the Cross.
Because it is measured from Pascha, Palm Sunday moves each year, always the Sunday before the Resurrection. The faithful carry palms and pussy-willow branches, a sign that they too go out to meet the King, and that His kingship is of a kind the world did not expect.