Source : Webster's Revised Unabridged Dictionary (1913)

Whitsun \Whit"sun\, a.
   Of, pertaining to, or observed at, Whitsuntide; as, Whitsun
   week; Whitsun Tuesday; Whitsun pastorals.

Source : WordNet®

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