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]