In Ulysses there are four references to Whit Monday: two by Bloom and two by Molly - usually in the context of Bloom’s bee sting on Whitmonday, 23 May 1904. In medieval times the period was a week-long holiday, and until 1973 Whit Monday was a Bank Holiday in Ireland. Pentecost is a movable feast celebrated fifty days after Easter.
Don Gifford annotates Molly’s use above as:
Whit Monday is a cursed day - Whitmonday, a bank holiday (23 May in 1904), follows Whitsunday (also called Pentecost), the seventh Sunday after Easter, when the descent of the Holy Spirit is commemorated (see Acts 2:1-6). The source of the superstition that Whitmonday is a cursed day is unknown.
Gifford should have extended the quote from Ulysses to include Molly’s “too”, as she seems to imply that Monday is cursed along with Sunday.
The superstitions surrounding Whitsuntide are well-documented in Irish folklore. In 1887 Oscar Wilde’s mother, Jane Francesca, Lady Wilde (who also used the pen name “Speranza”) published Ancient Legends, Mystic Charms, and Superstitions of Ireland, and introduced her section on Whitsuntide as follows:
Whitsuntide is a very fatal and unlucky time. Especially beware of water then, for there is an evil spirit in it, and no one should venture to bathe, nor to sail in a boat for fear of being drowned; nor to go a journey where water has to be crossed. And everything in the house must be sprinkled with holy water at Whitsuntide to keep away the fairies, who at that season are very active and malicious, and bewitch the cattle, and carry off the young children, and come up from the sea to hold strange midnight revels, when they kill with their fairy darts the unhappy mortal who crosses their path and pries at their mysteries.1 (vol. 1 "Festivals", p. 223-4)
Bryan J. Jones, in a correspondence entitled “Whitsuntide Fate” addressed to Folklore in 1904, also discusses Whitsuntide in reference to Lady Wilde’s work and personally confirms that “the beliefs she records still exist among the country people”, and continues to single out Whit Monday as a fateful day:2
Whit-Sunday does not appear to be an unlucky day, but Whit-Monday is, and so to a lesser degree are the Tuesday, Wednesday, Thursday, and Friday following. It is most dangerous to travel or boat on Whit-Monday, and children or animals born on that day are evil-tempered and likely to take some man's life. It was customary in Louth to bury a Whit-Monday foal or calf, but this method of breaking the charm is now almost forgotten, though such animals are still distrusted.
University of West Florida