Gattsu Saitama 2017 information here:
Halloween information is now in the Events section:
On the 17th of October there is a Halloween Festival and Gourmet Fair being held in the Saitama Super Arena from 10am to 9pm.
There are special events including a fancy dress competition. You need to be in full costume to enter the competition, just make up and/or a witches hat will not suffice. Registration is on the day, between 11am and 1pm.
The performances include many dance performances in the morning. The costume contest and cosplay show happens mid afternoon from about 3pm. That is followed by some more performances and music.
There is food and service booths as well as a designated kids space.
Saitama Super Arena is a 2 minute walk from JR Saitama shintoshin Station, about a 7 minute walk from JR Kitayono Station and there is parking available. The car park costs 400 円 for the first hour and 200 円 for every subsequent hour.
Saitama Super Arena website: https://www.saitama-arena.co.jp/
Event webpage: https://guts-minuma.net/