[Note: A more complete answer to this question was posted in 2014.]
Q: When did people start pronouncing “Halloween” as HOL-o-ween rather than HAL-oween?
A: I can’t tell you exactly when it happened, but it would appear that the HOL pronunciation has gained in popularity over the last half-century.
Contemporary dictionaries accept both the HAL (as in “hallowed”) and HOL (as in “holiday”) pronunciations for the first syllable of “Halloween.”
This is true for both The American Heritage Dictionary of the English Language (4th ed.) and Merriam-Webster’s Collegiate Dictionary (11th ed.).
No labels or usage notes in the two dictionaries indicate that one pronunciation is preferred over the other, though HAL is listed as the more common pronunciation.
“Halloween,” formerly written “Hallowe’en,” is short for “All Hallowed Even,” which is the evening preceding All Hallows, or All Saints Day (Nov. 1).
So one would think that etymologically the HAL pronunciation ought to be more historically accurate.
But my unabridged Webster’s New International Dictionary (2d ed.), dated 1956, notes that “an older pron. hol – cf. HOLIDAY – is still sometimes heard, but is generally considered now dialectal.”
This would suggest that the HOL pronunciation is an old one that has returned and become respectable in the last 50 years or so.