Beltane or Bealtaine is an ancient Gaelic holiday celebrated around May 1. This festival was celebrated in Ireland, Scotland and the Isle of Man. There were similar festivals held at the same time in the other Celtic countries of Wales, Brittany and Cornwall. See the Etymology section below for spellings and origins of the word Beltane.
Beltane is a cross-quarter day, marking the midpoint in the Sun's progress between the vernal equinox and summer solstice. The astronomical date for this midpoint is slightly later, around May 5 depending on the year. The festival marked the beginning of the pastoral summer season when the herds of livestock were driven out to summer pastures and mountain grazing lands. In modern Irish, Mí na Bealtaine, "month of the Bealtaine festival") is the name for the month of May. The name of the month is often abreviated to Bealtaine, but this strictly speaking only refers to the first day of summer Lá Bealtaine / (May 1) and the festival associated with that day and its eve. The lighting of bonfires on Oidhche Bhealtaine / "the eve of Bealtaine" on mountains and hills of ritual and political significance was one of the main activities of the festival. In ancient Ireland the main Bealtaine fire was held on the hill of Uisneach "the navel of Ireland", the reputed centre of the country, which is located in what is now County Westmeath. In Ireland the lighting of bonfires on Oidhche Bhealtaine seems only to have survived to the present day in parts of County Limerick as their yearly bonfire night. Another common aspect of the festival which survived up until the early 20th century in Ireland was the hanging of May Boughs on the doors and windows of houses and of the erection of May Bushes in farmyards, which usually consisted either of a branch of mountain ash or whitethorn which is in bloom at the time and is commonly called the May Bush in Hiberno-English.
Beltane has a complex etymology and a resultant variety of different spellings.
The word "Beltane" derives from the Irish Bealtaine in Scottish Gaelic it is spelt Bealtuinn; both from Old Irish Beltene "bright fire" from belo-te(p)niâ). Beltane was formerly spelt "Bealtuinn" in Scottish Gaelic; in Manx it is spelt "Boaltinn" or "Boaldyn".
In the word belo-te(p)niâ) the element belo- is allied to the English word bale (as in bale-fire), the Anglo-Saxon bael, and also the Lithuanian baltas, meaning "white" or "shining" from which the Baltic takes its name. The word Beltan in Wicca means "Fire in the sky." 
In Gaelic the terminal vowel -o (from Belo) was dropped, as shown by numerous other transformations from early or Proto-Celtic to Early Irish, thus the Gaulish god-names Belenos ("bright one") and his partner Belisama. Belenos was probably the same divinity, originally from belo-nos "our shining one", is also from the same source, as was Shakespeare's Cym-beline.
From the same Proto-Celtic roots we get a wide range of other words: the verb beothaich, from Early Celtic belo-thaich (to kindle, light, revive, or re-animate); baos, from Baelos (shining); beòlach (ashes with hot embers), from beò (originally belo) + luathach, "shiny-ashes" or "live-ashes".
Similarly boil, boile came from "fiery madness", through Irish buile, Early Irish baile: and boillsg (gleam); bolg-s-cio-; related to Latin fulgeo, "shine", English effulgent, Lithuanian blizgù, glance, shine, English blink (where the shine causes eyes to shut), Proto-Indo-European bhleg -> fulgeo (Grimm's law). In this way the Celtic tribe of Belgae in Northern France from which Belgium today takes its name, may derive from the same root. One of its tribes was called the Bellovaci. Some have suggested that the Ancient Irish "Fir Bolg" (i.e. "the Shining Ones") of Celtic mythology may have derived from the same word.
In Irish mythology, the beginning of the year for the Tuatha Dé Danann and the Milesians started at Bealtaine, great bonfires would herald in the Summer in the hope of good harvest, prosperity and well being to all. Early Gaelic sources from around the 10th century state that the Druids would create a need-fire on top of a hill on this day and rush the village's cattle through the fires to purify them and bring luck ("Eadar dà theine Bhealltainn" in Scottish Gaelic, "Between two fires of Beltane"). People would also go between the fires to purify themselves. This was echoed throughout history after Christianization, with lay people instead of Druid priests creating the need-fire. The festival persisted widely up until the 1950s, and in some places the celebration of Beltane continues today. A revived Beltane Fire Festival has been held every year since 1988 during the night of 30 April on Calton Hill in Edinburgh, Scotland and attended by up to 15,000 people (except in 2003 when local council restrictions forced the organisers to hold a private event elsewhere).
Beltane is a specifically Gaelic holiday, as other Celtic cultures, such as the Welsh, Bretons, and Cornish, do not celebrate it - though they celebrated or celebrate festivals similar to Bealtaine at the same time of the year.
Dwelly (1911) says: "In many parts of the Highlands, the young folks of the district would meet on the moors on 1st May. They cut a table in the green sod, of a round figure, by cutting a trench in the ground of sufficient circumferences to hold the whole company. They then kindled a fire, dressed a repast of eggs and milk of the constituency of custard. They kneaded a cake of oatmeal, which was toasted at the embers against a stone. After the custard was eaten, they divided the cake into as many portions as there were people in the company, as much alike as possible in size and shape. They daubed one of the pieces with charcoal, til it was black all over, and they were then all put into a bonnet together, and each one blindfolded took out a portion. The bonnet holder was entitled to the last bit, and whoever drew the black bit was the person who was compelled to leap three times over the flames. Some people say this was originally to appease a god, whose favour they tried to implore by making the year productive." (Dwelly, 1911, "Bealltuinn")
In neopaganism, the name Beltane or Beltaine is used for a sabbat, one of the eight solar holidays, which is celebrated on this day. Although the holiday uses features of the Gaelic Beltane, such as the bonfire, it bears more relation to the Germanic May Day festival, both in its significance (focusing on fertility) and its rituals (such as maypole dancing). Some Pagans celebrate High Beltaine by reenacting intercourse between the May Lord and Lady. Gerald Gardner, one of the principal originators of the Wiccan religion, referred to the holiday as May Eve.
Among the neopagan sabbats, Beltane is a cross-quarter day; it is celebrated in the northern hemisphere on May 1 and in the southern hemisphere on November 1. Beltane follows Ostara and precedes Midsummer (see the Wheel of the Year).