Chalice Centre (where I found the charming image above) has an overview of how Samhain was observed in pagan times. Hearth fires were extinguished and re-lit from a sacred source, and people danced around great bonfires into which goods sacrificed to the gods were cast. The reverence for fire undoubtedly dates back to the Aryan conquests of more than five millennia ago, and is found in many cultures sharing the same origin. Fire was similarly held divine in Zoroastrian Persia, for example, and many modern-day Iranians continue to observe the fire-festivals in defiance of the mullahs' dour edicts of condemnation.
In the British Isles, similarly, Samhain rituals survived the coming of Christianity. As it did with so many other traditional European sacred days, the new alien faith out of the Middle East sought to Christianize Samhain and co-opt it, rather than to eradicate it entirely. In the seventh century Pope Boniface IV declared the first day of November to be "All Saints' Day", and the preceding night became "All Hallows' Eve", from which the name "Halloween" is derived. Yet the bonfire dances continued -- in some parts of Britain, as late as the early twentieth century.
Samhain was observed under different names in various Celtic lands. The practice of "apple magic", mentioned at the end of the Chalice Centre post linked above, survived in the Cornish festival of Calan Gwaf or Allantide (found via Mendip) and in more diluted form in the game of apple-bobbing. Halloween costumes and trick-or-treating, which play such a central role in our modern concept of Halloween, are foreshadowed in the festival of Hop-tu-Naa on the Isle of Man, a small island between Britain and Ireland.
Modern Christian fundamentalists remain profoundly suspicious of Halloween, and with good reason. Unlike Christmas, Halloween was never successfully infused with Christian significance; to this day it remains, in its symbolism and imagery, the most boldly pagan observance in the Celtic- and English-speaking world.
[Re-posted from Halloween 2010.]