In the 20+ years I've been here, Halloween has gone from a virtually unknown holiday with strange gaijin in costumes on the train, to what is rapidly approaching mainstream status, even to the point of being printed on calendars.
One of my students, a 3-yr-old girl, was so excited at getting her new costume that she wore it to class. On October 2nd....
And Japanese retailers love the holiday. You can get Halloween versions of virtually every snack, even ones you wouldn't normally associate with Halloween, like seaweed-flavored potato chips. The 100-yen stores carry all kinds of decorations and costumes (her costume above came from one), there are even events at malls.
The local mall asked me to bring some students for an event yesterday, where the kids played Monster Bingo, then paraded around giving snacks to the customers.