The Celts established the festival of Beltane (Beltaine) to begin summer.

An excerpt from the article 39 facts about Ireland

This festival is also known as the "fires of Belenos" or Cethsamhain - "against Samhain" - marking the end of the harvest season and the beginning of winter or the "darker season" (traditionally it is celebrated from October 31 to November 1, because the Celtic day began and ended at sunset).

Beltane fell on the night from April 30 to May 1. The fires in the houses were then extinguished and rekindled with a new fire lit by the druids (ancient Celtic priests). An important element of the Celtic Beltane ritual dedicated to fertility was the ceremonial burning of a large effigy called the wicker man.

Nowadays, the holiday is celebrated as the Beltane Fire Festival.