What is Palm Sunday? Here's an explanation from Catholic.org
The sixth and last Sunday of Lent and beginning of Holy Week, a Sunday of the highest rank, not even a commemoration of any kind being permitted in the Mass. In common law it fixes the commencement of Easter duty.
The Roman Missal marks the station at St. John Lateran (see STATIONS) and before September, 1870, the pope performed the ceremonies there.
The Greeks celebrate the day with greatsolemnity; they call it kyriake or heorte ton baion or heorte baiophoros or also Lazarus Sunday, because on the day before they have the feast of the resuscitation of Lazarus. The emperors used to distribute branches of palm and small presents among their nobles and domestics. The Latin liturgical books call it Dominica in Palmis, Dominica or Dies Palmarum. From the cry of the people during the procession the day has received the name Dominica Hosanna or simply Hosanna (Ozanna). Because every great feast was in some way a remembrance of the resurrection of Christ and was in consequence called Pascha, we find the names Pascha floridum , in French Pâques fleuries , in Spanish Pascua florida , and it was from this day of 1512 that our State of Florida received its name ( Nilles, II, 205).
From the custom of also blessing flowers and entwining them among the palms arose the terms Dominica florida and dies floridus. Flower-Sunday was well known in England, in Germany as Blumensonntag or Blumentag , as also among the Serbs, Croats, and Ruthenians, in the Glagolite Breviary and Missal, and among the Armenians.
Since this Sunday is the beginning of Holy Week, during which sinners were reconciled, it was called Dominica indulgentioe, competentium , and capitilavium from the practice of washing and shaving of the head as a bodily preparation for baptism.