4 real Seasons a year. SO dress accordingly.
Don't bring more than you need. It sucks having to carry around a huge suitcase on the way here. If you have a lot of heavy luggage, it's better to send your luggage to your address by one of the delivery companies located at the airport. However, delivery will take a few days (2 or 3). In winter: You MUST have winter clothes. This place gets pretty cold and includes wind chill.
In spring: You can dress more comfortably.
Slacks and short sleeves may be ok during the day. Have a jacket at night.
In summer: You can look forward to shorts, t-shirts and humidity in July & August. It rains in June or July.
In fall: It gets cold and it rains. And the wind is cold. Bring toothpaste with flouride. Japanese toothpaste doesn't have floride. Bring deodorant if you need it. Bring the right kind of shoes for the right kind of season.
In winter you'll need boots with water repellent. If you're going to be here in January or February, you might want to consider buying (or bringing) an electric blanket for those freezing cold winter nights.

Niigata Starbucks Shichikuyama location (3)
We arrive at 6:30, 1 hour before the show begins
looking down on a Bandai mainstreet area
Robots mimic what used to be done by workers