Halloween/All Saints' Eve has been corrupted and co-opted with a great deal of commercialism, mysticism, and Modern Neopagan/Wiccan influence. But the origins and purpose of the holy day are uniquely Christian. It did not originate in Celtic practices.
And despite the many articles and documentaries which may make the claim: Halloween does not come from pagan origins, witchcraft, or the occult. It was in no way attached to these kinds of belief systems in its origin. Only in the last 2 centuries have these other religious movements started to claim the day and integrate their own rituals with Halloween/All Saints.
This link has more full discussion with documentation:
(All Saints' Eve, All Saints' Day: Origins and Samhain-ization)
Here is a brief summary:
The development of Halloween as a Church Holy Day began over 1,000 years prior to any known evidence of a Celtic day or festival with the name Samhain (pronounced "Sow-in"). The All Saints' day practices originated in non-Celtic areas and were widespread before the Celtic churches adopted the practice. Annual dates varied from region to region and became fixed on Nov 1 in the 8th century. The Celtic churches adopted this date from Rome and the Frankish kingdoms. This date was adopted by the Celtic Church nearly 2 centuries before there is any known mention of Samhain in Celtic literature.
The Reformation and Halloween
All Hallows' Eve/All Saints' Day is an historically significant day in the western Church not only because of the Church feast, but also because of the events that took place in Wittenberg in 1517. This year we celebrate the 500th anniversary of the Reformation. It is more than a little ironic that the popular modern corruption of All Saints' Eve centers on fear, the supernatural, death, and the state of those who have died. This topic is addressed at this link: All Hallows' Eve in the Mediaeval Church and the Reformation.
If You're Really Into Learning More
I've put together a timeline/slideshow that highlights the development of Halloween, Samhain, and Celtic influences. It is available here.
And I have a short article dealing with some specific claims about a prehistoric megalithic tomb in Tara, Ireland. It is often claimed that this tomb must be evidence that Samhain is ancient, even pre-Celtic. But the claims do not really hold up. The article is: Samhain and The Mound of Hostages, Tara, Ireland.