All Hallows' Eve

It's concept is brilliant, it's traditions divine.
Everyone can by anyone and no one. Halloween is the day that world becomes a stage and we are, literally, all the players. Dressed in costume, perhaps hidden by a mask you are given the leeway to act out whatever part you have secretly always wanted to be. Or maybe you dress as the personification of something you are every day. But the point is that nobody can be judged for what they wear, or how they act.
Every day people dress up and put on a "mask", or maybe they put on several, depending on the circumstances. We all act our way through life, never truly revealing who we really are to the general public. And Halloween celebrates this.
Originally, Halloween was the day that the line between living and dead blurred, now it is the day when the line between fantasy and reality blurs. Freudian theory would have me argue that just as ancient Halloween celebrators let things that were long buried wander among the living, we now let the fantasies of our unconscious out and put them on display. We become the character we could be in our mind for an evening and have no concern for what society may think of us - because they're dressed equally as outrageously.
Personally, the concept of dressing up was just always incredibly appealing. As a child, it was always the costume that was attractive and the candy was just an added bonus. Carving pumpkins? Messy and fantastic. Scaring people? Darkly fun. Letting out your inner demons? Necessary.