Japan has four distinct seasons, similar to those on the east coast of the United States. Summer begins in June and introduces the rainy season; typhoon season comes in late August and September — carry an umbrella during these periods. Expect that some days will be hot and humid.
Autumn is the perfect time to visit as the days are cooler and the changing season is reflected in the trees. Winter is cold, with some potential for snow in Tokyo — more Northern areas receive a more traditional snowfall. Spring, of course, heralds the arrival of cherry blossoms and perfect weather for tourists.
With an exception of winter months, heavy clothing is generally not required. During autumn and spring months, anticipate some temperature fluctuations by bringing clothing that can be layered.