http://magickalmusings.net/wicca/wheel/beltain.php

May Day ushers in the fifth month of the modern calendar year, the month of May. This month is named in honor of the goddess Maia, originally a Greek mountain nymph, later identified as the most beautiful of the Seven Sisters, the Pleiades. By Zeus, she is also the mother of Hermes, god of magic. Maia's parents were Atlas and Pleione, a sea nymph.

   The old Celtic name for May Day is Beltane (in its most popular Anglicized form), which is derived from the Irish Gaelic "Bealtaine" or the Scottish Gaelic "Bealtuinn", meaning "Bel-fire", the fire of the Celtic god of light (Bel, Beli or Belinus). He, in turn, may be traced to the Middle Eastern god Baal.

   Other names for May Day include: Cetsamhain ("opposite Samhain"), Walpurgisnacht (in Germany), and Roodmas (the medieval Church's name). This last came from Church Fathers who were hoping to shift the common people's allegiance from the Maypole (Pagan lingham - symbol of life) to the Holy Rood (the Cross - Roman instrument of death). Incidentally, there is no historical justification for calling May 1st "Lady Day". For hundreds of years, that title has been proper to the Vernal Equinox (approx. March 21st), another holiday sacred to the Great Goddess. The nontraditional use of "Lady Day" for May 1st is quite recent (within the last 15 years), and seems to be confined to America, where it has gained widespread acceptance among certain segments of the Craft population. This rather startling departure from tradition would seem to indicate an unfamiliarity with European calendar customs, as well as a lax attitude toward scholarship among too many Pagans. A simple glance at a dictionary ("Webster's 3rd" or O.E.D.), encyclopedia ("Benet's"), or standard mythology reference (Jobe's "Dictionary of Mythology, Folklore & Symbols") would confirm the correct date for Lady Day as the Vernal Equinox.

   By Celtic reckoning, the actual Beltane celebration begins on sundown of the preceding day, April 30, because the Celts always figured their days from sundown to sundown. And sundown was the proper time for Druids to kindle the great Bel-fires on the tops of the nearest beacon hill (such as Tara Hill, Co. Meath, in Ireland). These "need-fires" had healing properties, and sky-clad Witches would jump through the flames to ensure protection.