The dates of religious holidays are, and always have been, subject to change... I don't want to call them arbitrary... but... I mean 'Christmas' was decided by a council to lie officially on Dec. 25th in 386ad. Was that (dec. 25) the date of Jesus' birth? Maybe, maybe not.
So is halloween just a made up holiday? Well, you could argue that, and I'd argue that the dates of all other holidays are just made up too; and you'd probably say 'no way, you can't do that!' and I'd say 'you're right, because that would be ignoring the meaning'.
Halloween (Hallowe'en, All Hallow's Eve, All Hallowed Soul's Eve, All Saint's Eve), while you could argue on a base level is nothing but Paganism and whatnot, I would contend that while you would be correct in saying the warding of lost spirits is purely mysticism, there is an honouring of the departed going on. There's hardly anything about honouring the memory of your ancestors, countrymen, etc, that is fundamentally non-Christian. After all, the only reason we still have this holiday is because the Catholic church appropriated these customs as a propaganda technique when conquering scattered tribes and cultures of Europe.
Not that our culture encourages the informing of one's self on the histories of what are now primarily secular events. But anyway.
Devil's night, October 30th, is just a bullshit excuse for mischief (by mischief, I mean vandalism) with no real meaning to it, however. Feel free to condemn that as much as you like.