This is one of the most important places I think Japan should visit, not one of them. It usually starts in late June to early September. The best time to go is midsummer. It's very sunny in the mountains and very cold at night. Mountaineering should be well equipped. The top of the mountain is very steep, remember to wear gloves. If you choose to climb the next day to see the sunrise, you'd better go to Octahedral or Octahedral Half to find a place to live the day before. (Japanese official website can be booked, earlier.) Remember to take the lights with you. Many mountain climbers in the early morning need not worry one by one.