Calan Mai is a Welsh festival on the first day of May, transitioning the year from the winter months to the summer months. Like most Gaelic festivals (such as Beltane in Ireland), the celebration for Calan Mai actually started on the eve before May 1st, relating more to the lunar cycle and the liminal space before the day being more powerful at night. Bon fires were lit at night, costumes and revelry took place, and cattle were driven between the fires to ensure their safety for the grazing months.
"It was one of the three great ‘spirit nights’ (ysbrydnos) of the year. These were thought to be the three nights of the year that the veil to the spirit world was at its thinnest and people would use this to foretell who their true love would be. (The other two ysbrydnos nights being St John’s Eve (24th June) and Halloween (31st October)."