Halloween or Hallowe'en?

I seem to remember when I was a kid that we used to celebrate Hallowe'en but somewhere along the way the apostrophe disappeared and we now celebrate Halloween.

I know the apostrophe is to mark the elision of the letter "v" because we are talking about All Hallows' Eve(n). But I wonder exactly when it went and who decided it would go.

The Ninth Edition of the Concise Oxford Dictionary (1996) gives the apostrophised spelling. The Cambridge Dictionary of American English (2000) gives both alternatives, with the first (and presumably more common) spelling Halloween, as does the Cambridge Advanced Learner's Dictionary.

Let me check out a few more dictionaries and I'll get back to you.