Japan

Winter: Dec-Mar (Avg. 42F) Generally cool and dry
Spring: Mar-May (Avg. 55F) Warmer, more humid
Summer: Jun-Aug (Avg. 77F) Hot, humid, unstable
Fall: Sept-Nov (Avg. 63F) Dry, stable, clear
Seasons are an important part of Japanese life as they highlight the fleeting aspect of our existence. The four seasons are marked in art, customs, festivals, and daily life. The best times to visit Japan are the stable seasons of spring (Mar. to May) and autumn (Sept.-Nov).
Spring receives many visitors due to the Sakura (mid-March to mid-April), celebrating Japan's famous cherry blossoms. Late spring can be warm and humid.
Autumn is arguably one of the best times to visit Japan due to the pleasantly cool and dry temperatures, and lack of crowds. The changing leaves contrast beautifully with the clear blue skies.
With the exception of Hokkaido and mountain top resorts, most of Japan is very hot and humid during the summer (Jun.-Aug). This is also the rainy season, so remember to pack your umbrella. Be prepared for the occasional thunderstorm during typhoon season (End of Aug. to Sep).
Winter (Dec.-Mar.) is marked by snow in most of Japan (except Tokyo), especially in the mountains where skiing is superb. Hot-spring resorts are very popular with Japanese and foreign tourists during this time. Southern areas like Okinawa and Kyushu experience warmer mild winters.
Keep in mind that peak holiday periods, particularly Golden Week (late April-early May), and the mid-August O-Bon (Festival of the Dead), are extremely popular and bring large crowds. Likewise, everything in Japan basically shuts down during Shgatsu (New Year).