Halloween is not a Japanese tradition, but in Tokyo you can't escape the jack-o-lanterns, witches' hats, ghosts, and pumpkin flavored everything - it has recently exploded in popularity. There are costume parties in night clubs and bars, and if you know when and where to look you can find the odd "secret" trick-or-treat friendly matsuri where local businesses will provide candies to children in costume. Generally you will not see any door-to-door trick-or-treating.
A warning to those wishing to dress up in Japan for Halloween, people will freak out if you're dressed up as a crazed chainsaw killer. In fact just last year police and special riot squads were dispatched to approach a man in Kumamoto wearing a mask and carrying a convincing toy chainsaw (source in Japanese). So swords, guns, etc - probably will earn you an armored bus encounter.
On the candy front, this month we'll send a tiny pumpkin flavored Kit-Kat - because we guess you may not really need another pumpkin flavored anything but it's still neat just to sample.