Bealtaine (or Beltane) is the Gaelic name for both the month of May and a festival that traditionally took place throughout Ireland on the first day of May. The festival was also celebrated in Scotland and Wales.
Beltane marks the beginning of the summer season in Ireland. At the end of the day on May 1, the communities would light a big bonfire on top of a hill, all across Ireland. The bonfires were lit to drive out evil spirits and cleanse the community to prepare for the joyous days of summer and hopefully a bountiful harvest in the fall. All of the residents of the community would bring their own torch to the bonfire to get new fire, take it home, and re-light their own hearths as a symbol of a new beginning. Because the festival involved lighting fires, the first day of May was called "Laa Buidhe Bealtaine (Bright May Day).
The tradition of lighting bonfires continues in parts of Ireland and Scotland, and among some groups in the Irish diaspora.
Another part of the festival was the hanging of Mayflowers, or flower arrangements, on the doors and windows of the homes, and the erection of a May bush which was decorated with flowers and ribbons (similar to May poles).
Here is a video of a recent Beltane celebration.