We celebrated Halloween while in the US, thinking we wouldn't be doing anything in Japan. We knew our ward was having a Trunk or Treat activity, but it was while we were gone. I also heard rumors that certain Tokyo neighborhoods with a high percentage of ex-pats celebrated, but that was about it.
After sending the kids to school the morning after we got home (since they woke up at 5am anyways), we sent them off to school. When they came home they had a flyer about a Halloween party at their school! The kids were so excited.
I will say that the Japanese are just starting to adopt the Halloween tradition, but they don't quite understand it yet. In fact, on the flyer they included a brief description of what Halloween even was. :) Most kids showed up in costumes (a lot were witches because the 100 yen aka Japanese dollar store, was selling witch hats and capes) and the teachers handed them a bag of candy. That was it. The kids played on the playground and looked at each others costumes for about an hour. It was fun taking pictures and seeing all of the cute kids, but like I said...they just don't quite get it yet. :)