Is there anything in Jewish or Christian law against a masquerade party? That's a retorecal question because the answer is.. "Of course not."
Well, that is all that Halloween is about a masquerade, having harmless fun, that's it, nothing else, no deeper meaning, if it did mean something 800 years ago or whatever that is long gone now, ask any adult at a Halloween party, ask any kid, not one will say "I'm celebrating a pagan holiday", they'll all say, "fun day, costume, candy". That is the only meaning of it in the contemporary world, and that is what is important, what it means now.