The Collaborative International Dictionary of English v.0.48:
Whittuesday \Whit"tues`day\, n. (Eccl.) The day following Whitmonday; -- called also Whitsun Tuesday. [1913 Webster]WordNet (r) 3.0 (2006):
Whit-Tuesday n 1: the day after Whitmonday [syn: Whit-Tuesday, Whitsun Tuesday]