What is "nettlemas"

Nettlemas is an ancient Irish holiday, celebrated on the first of May. In Irish, it is called Féile na Neantóg. The preceding day, 30 April, is Nettlemas night. Nettlemas night is sometimes referred to as only Nettlemas, which might cause confusion about the date of the holiday. On Nettlemas night, boys walked in the streets stinging one another with stinging nettles.

Noted anthropologist James Mooney mentions it in his book, The Holiday Customs of Ireland (1889).