Calan Mai ( "Calend (first day) of May") or Calan Haf ( "Calend of Summer") is a May Day holiday of Wales held on 1 May. Celebrations start on the evening before, known as May Eve, with bonfires; as with Calan Gaeaf or November 1, the night before is considered an Ysbrydnos or "spirit night" when spirits are out and about divination is possible. The tradition of lighting bonfires celebrating this occasion happened annually in South Wales until the middle of the 19th century.