Article by Grammar Girl appeared on The Quick and Dirty, on 26th October, 2011
What’s your take on the apostrophe in Hallowe’en? To use or not to use? One early spelling of Halloween was all hallows’ even, in which even meant evening. The all and s got dropped, hallows’ and even became a closed compound, and the apostrophe took the place of the v, giving us Hallowe’en—just one of many transitional spellings along the way to Halloween, which the Oxford English Dictionary shows as first appearing in 1786.
We celebrate Halloween on the last day of October because the holiday was originally tied to the Celtic calendar, in which November 1 was the start of the new year. The Catholic Church later adopted November 1 as All Saints’ Day, and October 31 is also sometimes called All Saints’ Eve.