Climates (4 seasons)

Spring, Summer, Autumn, Winter

In the spring, most of all Japanese go to see a cherry blossom and cerebrate the spring.
Strongly recommend to visit 1st week of April, if you want to see the cherry blossom.

Between Spring and summer, there is rainy season (June).

In summer, hot and humid (50~60%), you could see and enjoy fireworks. So many Fireworks event will be hold during summer and anywhere in Japan. Mainly weekend. Japan's Fireworks should be nice, but have to be prepared for heavy traffic.

Autumn, best season for sightseeing. Enjoy autumn leaves, food and hot spring.
To see autumn leaves, visit Temple and shrine, best combination.

Winter is also good for fish food, able to see snow in mind/north.