Hallowe'en is the name given in English to the evening of October 31

Just as the evening of December 24 has its own name, Christmas Eve, so does the Catholic holy day which falls on November first. That feast is called All Saints Day. Now if English were a consistent language, October 31 would be called All Saints Day Eve, but it isn't. It isn't because what we now call All Saints Day had a different name in the past. It was called All Hallows Day, the word hallow meaning blessed. The evening before was then called All Hallows' Evening.

English likes to shorten words when it can, and the first word it shortened was "evening," first to even' and then to eve. The next thing that happened to the name was the elimination of the word all. Soon people could be talking about Hallows Eve. English speakers could have gone there, but that would have meant that the name would be consistent with Christmas Eve, and that wouldn't have been confusing enough for English learners.

The problem with using Hallows Eve is that there was already an abbreviation for the name of the day: Hallows Even'. Dropping the s and the v, one approaches the name I learned, Hallowe'en. Today the apostrophe has disappeared and we have left the modern spelling of Halloween.

Halloween or Hallowe'en (a contraction of "All Hallows' Evening"), also known as Allhalloween, All Hallows' Eve, or All Saints' Eve, is a celebration observed in many countries on 31 October, the eve of the Western Christian feast of All Hallows' Day. The night before Alholowmesse was called" All Hallows Even (evening)" which was eventually shortened to "Hallowe'en" until it just became "Halloween" in the 20th century.