Beltane is an old Gaelic holiday celebrated at the traditional beginning of summer, on May 1. It has become well-known again as a holiday adopted by Wiccans, who have made it one of their eight major holidays, based on the movement of the sun.
Why eight holidays? The movement of the sun is marked by four distinct movements: the spring and autumn equinoxes, and the summer and winter solstices - and the midpoint between each alternating equinox and solstice is also a holiday. So, Beltane is a "cross-quarter holiday," meaning that it is halfway between an equinox (the spring equinox) and a solstice (the summer solstice). As an astronomical holiday, the date can vary from year to year, but Beltane has become traditionally associated with the first day of May, with Beltane Eve celebrated on April 30.
The holiday that is paired with Beltane as a seasonal pair is Samhain, which comes between the autumn equinox and the winter solstice. Like Samhain, Beltane is a holiday traditionally associated with the other-world. One of the traditional festivities associated with Beltane is the lighting of bonfires on hilltops.
The lighting of bonfires is also associated with the festival of Walpurgisnacht, the Eve of Saint Walburga Day. The Walpurgisnacht festival has become associated with witches and witchcraft, as seen, for example, in Goethe's Faust. In more recent literature, J.K. Rowling has said that the "Death Eaters" in the Harry Potter books were originally called the "Knights of Walpurgis."
Walpurgis Night is one of the major holidays in Sweden (Valborgsmässoafton) where it is celebrated with bonfires. It is also a major holiday in Finland.
Here is a Walpurgis Night bonfire from Sweden: