This blog is for tourists coming to Japan. 
The best times to visit Japan
are the climatically stable seasons of spring 
(March to May) and autumn 
(September to November).
Traveling during either winter or summer is a mixed bag- midwinter (December to February) weather can be cold, particularly on the Sea of Japan coasts of Honshu and in Hokkaido, while the summer months (June to August) are generally hot and often humid. June is also the month of Japan’s brief rainy season, which in some years brings daily downpours and in other years is hardly a rainy season at all.
If you’re heading to the southern island groups like Okinawa, the islands of southern Kagoshima-ken, the Izu-shoto, or the Ogasawara-shoto, keep in mind that typhoons can wreak havoc with your travel plans. They can strike anytime between June and October, with most making a landfall in August, September and early October.
Also keep in mind the peak holiday seasons, particularly Golden Week (late April to early May), which incorporates four major Japanese holidays, and the mid-August O-Bon (Festival of the Dead), are extremely popular for domestic travel and can be problematic in terms of reservations and crowds. Likewise, everything in Japan basically shuts down during Shogatsu 
(New Year period).
All that said, it is worth remembering that you can comfortably travel in Japan at any time of year. Indeed, if you come in winter or summer, you’ll escape the crowds and probably get better for your plane ticket and accommodation.