It's the time of year when, every year, I get annoyed by the Christians at one extreme and Neo Pagans at other extreme posting silly things about the ancestry of Halloween and the ancient Celtic festival of Samain. I thought I'd try to forestall some of the less factual assertions by a list of bullet points about what we actually know about Samain and the Celts, since Samain is one of the most popular Neo Pagan festivals or feast-days.
In 609 Pope Boniface IV designated November 1 All Saints' Day. The night before, October 31 consequently became All-Hallows Eve, or Halloween. In A.D. 1000, the church made November 2 All Souls' Day, a day to honor the dead. There are references to both feast days earlier in some places, but these are the dates of official sanction.