Halloween isn't typically celebrated in Japan. It's not a traditional part of the culture, though dressing in costume is obviously a much loved hobby for many Japanese. It's not really surprising that after appropriating Christmas (it ain't about Jeebus in Japan, I can tell you that much) the next holiday to gain momentum would be Halloween. Who doesn't love an excuse to dress up and have fun?!
While I encountered some festively dressed folks on my way to work, the real Halloween hijinks were to be had at a party with coworkers afterwords.
Several of my coworkers were Americans as well, so we had our own version…which involved watching Nightmare before Christmas in Japanese. My favorite Japanese version song was "What's This?!", though, as Jyakku shouting "NANDA?! Nani-?!" repeatedly amuses me.