Technically....they shouldn't...because pretty sure the whole Halloween practice is to get rid of the devil rather than to celebrate it...but times have changed long since then so you could say that it is mostly invalid. Plus Christians don't do Halloween from the religious retrospect it's just a opportunity to dress up and have fun and get all the candy too!
But ideally it's not meant to be allowed, well says the Bible anyway, but the Bible says a lot of things and people still do the opposite.