May (Gaelic calendar). The name is derived from the feast of Bealtaine, which was celebrated on, or near, May 1st:
In Irish mythology, the beginning of the summer season for the Tuatha Dé Danann and the Milesians started at Bealtaine. Great bonfires would mark a time of purification and transition, heralding in the season in the hope of a good harvest later in the year, and were accompanied with ritual acts to protect the people from any harm by Otherworldly spirits, such as the Aos Sí. Like the festival of Samhain, opposite Beltane on October 31st, Beltane was also a time when the Otherworld was seen as particularly close at hand.