The term Halloween (also spelled Hallowe’en) is shortened from All Hallows’ Even – e’en is a shortening of even, which is the origin of the words “evening” and “eve”. This is ultimately derived from the Old English Eallra Hālgena ǣfen. It is now known as “Eve of” All Saints’ Day, which is November 1st.

In the 800s, the Church measured the day as starting at sunset, in accordance with the Florentine calendar. Although All Saints’ Day is now considered to occur one day after Halloween, the two holidays were once celebrated on the same day.

Read more:en.wikipedia.org/wiki/Halloween#History