Beltane has been celebrated in many different forms for centuries. The most commonly thought of one (the one wiccans and pagans use today) is the
Celtic fertility rite which took place on or around May 1st.
Nowadays the tradition is used by wiccans to celebrate the union of the goddess and the god. It's when the goddess changes from maiden to warrior and the god stops being her child (incest is lovely, eh?). This is also where she becomes pregnant with the god's child, who will be the reincarnation of the god after his death. (But please do note that just because someone's pagan doesn't make them wiccan).
For many pagans this is the best holiday of the year. Picnics, dancing, and lots of sex. There's a Maypole to dance around, a fire to leap over, and faeries to leave presents for, all in the name of fertility for the earth.
This fire festival is traditionally the exact opposite of Samhain.