What is the correct spelling or Halloween?

You pronounce it like "Hollow-een." So the "a" is actually pronounced as an "o." :) Have a Happy Halloween!

UPDATE: The above is incorrect. The origin of "Halloween" is "All Hallows' Eve," not "All Hollows' Eve," so the correct pronunciation is "HAL-oween" (Hallow-een), not "HALL-oween" (Hollow-een).