The Collaborative International Dictionary of English v.0.48:
Whitsun \Whit"sun\, a. Of, pertaining to, or observed at, Whitsuntide; as, Whitsun week; Whitsun Tuesday; Whitsun pastorals. [1913 Webster]WordNet (r) 3.0 (2006):
Whitsun n 1: Christian holiday; the week beginning on Whitsunday (especially the first 3 days) [syn: Whitsun, Whitsuntide, Whitweek]