It was one of the four major festivals in the Celtic calendar - Imbolc, Beltane, Lughnasad and Samhain. Beltane, a fire festival, celebrates the coming of summer(well, it is summer in the northern hemisphere, anyway; I hear that some neo pagans on this end of the world celebrate it in November instead of May, which makes sense as a seasonal thing, but must throw all of them out of synch.)
A lot of fantasy writers have made use of this and other festivals - I know I have. I especially used Samhain to create a very dramatic scene near the end of Wolfborn, with a huge storm, an attempt to kill the king, two werewolves, the good guy and the villain, slugging it out and the Wild Hunt making an appearance. I also had a Beltane scene which started the problems of poor Lord Geraint, who spends most of the rest of the novel in wolf shape due to a major mistake he makes on May Day itself.
Seasons were important in a society in which if your animals died or your crop failed you could be in for a long, hungry winter. Winter was the time when you couldn't do much anyway, and you were looking forward to seeing the sun come back.
Some of the celebrations went on during the time of Christianity, though probably tamer.
Anyone who has read Harvest Home by Tom Tryon(who wrote Stephen King-type stuff back when Stephen King had barely started) will know about the Sacred King. I'll leave you to check it out.