The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

Palm Sunday
    n 1: Sunday before Easter

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "Palm Sunday":
   Advent, Allhallowmas, Allhallows, Allhallowtide, Annunciation,
   Annunciation Day, Ascension Day, Ash Wednesday, Candlemas,
   Candlemas Day, Carnival, Christmas, Corpus Christi, Easter,
   Easter Monday, Easter Saturday, Easter Sunday, Eastertide,
   Ember days, Epiphany, Good Friday, Halloween, Hallowmas,
   Holy Thursday, Holy Week, Lady Day, Lammas, Lammas Day, Lammastide,
   Lent, Lententide, Mardi Gras, Martinmas, Maundy Thursday,
   Michaelmas, Michaelmas Day, Michaelmastide, Pancake Day,
   Passion Week, Pentecost, Quadragesima, Quadragesima Sunday,
   Septuagesima, Shrove Tuesday, Trinity Sunday, Twelfth-day,
   Twelfth-tide, Whit-Tuesday, White Sunday, Whitmonday, Whitsun,
   Whitsunday, Whitsuntide, Whitweek