The Collaborative International Dictionary of English v.0.48:
Whitmonday \Whit"mon`day\, n. (Eccl.) The day following Whitsunday; -- called also Whitsun Monday. [1913 Webster]WordNet (r) 3.0 (2006):
Whitsun Monday n 1: the day after Whitsunday; a legal holiday in England and Wales and Ireland [syn: Whitmonday, Whitsun Monday]