The Collaborative International Dictionary of English v.0.48:

Whitsuntide \Whit"sun*tide`\, n. [Whitsunday + tide.]
   The week commencing with Whitsunday, esp. the first three
   days -- Whitsunday, Whitsun Monday, and Whitsun Tuesday; the
   time of Pentecost. --R. of Gloucester.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Whitsuntide
    n 1: Christian holiday; the week beginning on Whitsunday
         (especially the first 3 days) [syn: Whitsun,
         Whitsuntide, Whitweek]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "Whitsuntide":
   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, Palm Sunday,
   Pancake Day, Passion Week, Pentecost, Quadragesima,
   Quadragesima Sunday, Septuagesima, Shrove Tuesday, Trinity Sunday,
   Twelfth-day, Twelfth-tide, Whit-Tuesday, White Sunday, Whitmonday,
   Whitsun, Whitsunday, Whitweek