The word Samhain is actually the old Celtic word for the month of November and in particular the first three days of the month. These three days were to mark the end of the summer days and the harvest and the beginning of the days of winter. In fact October 31st is still considered the traditional first day of winter in Ireland.
In the old Celtic belief this three-day festival is also the time when the worlds of humans and the dead are closest and the spirits of our ancestors move among us. With the rise of Christianity, quite a few of the old holiday dates were utilized to maintain familiarity for the people so they could be easily swayed over to the new way of doing things.
Thus November 1st became All Hallows' Day, November 2nd All Saints Day, and eventually October 31st All Hallows' Eve. Many Catholic countries still celebrate all three days with the final being the Day of the Dead, a day in which to venerate those that came before.
In the past few years there has been an upsurge in popularity in things associated with witchcraft. Television shows like Buffy The Vampire Slayer and movies like Practical Magic and The Craft have served to sensationalize the arcane side of these practices without exploring any of the history behind the beliefs.
Most of the blame for that should be laid on the shoulders of these so-called witches and their mysteries. Somehow it was decided the beliefs should be kept from the general public and that their holy book, The Book Of Shadows, be forbidden to all but those initiated in the craft. This has ensured that no one knows anything about them or their practices.
Oh sure, it makes them all important and mysterious sounding: you have to be initiated into the belief if you want to learn their secrets because they're obviously too potent for just anybody to mess around with. I wonder if they have a secret handshake as well so they can differentiate between those who claim to be Wiccan and those who have had the mysteries revealed to them.