Official language is Japanese. Japan uses metric system (kilograms, centimeters, °C). Time zone is UTC+09:00. Japan uses 100V, 50/60Hz with sockets and plugs Type A and Type B.
The Japanese are proud of their four seasons (and an astonishing number of them are firmly convinced that the phenomenon is unique to Japan), but the tourist with a flexible travel schedule should aim for spring or autumn.
Spring is one of the best times of year to be in Japan. The temperatures are warm but not hot, there's not too much rain, and March-April brings the justly famous cherry blossoms (sakura) and is a time of revelry and festivals.
Summer starts with a dreary rainy season (known as tsuyu or baiu) in June and turns into a steambath in July-August, with extreme humidity and the temperature heading as high as 35°C. Avoid, or head to northern Hokkaido or the mountains of Chubu and Tohoku to escape. The upside, though, is a slew of fireworks shows (花火大会 hanabi taikai) and festivals big and small.
Autumn, starting in September, is also an excellent time to be in Japan. Temperatures and humidity become more tolerable, fair days are common and fall colors can be just as impressive as cherry blossoms. However, in early autumn typhoons often hit the southern parts of Japan and bring everything to a standstill.
Winter is a good time to go skiing or hot-spring hopping, but as some buildings lack central heating, it's often miserably cold indoors. Heading south to Okinawa provides some relief. There is usually heavy snow in Hokkaido and northeast Japan due to the cold wind blasts from Siberia. Note that the Pacific coast of Honshu (where most major cities are located) has milder winters than the Sea of Japan coast: it may be snowing in Kyoto while it is cloudy or sprinkling rain in Osaka, an hour away.
Hokkaido Northernmost island and snowy frontier. Famous for its wide open spaces and cold winters.
Okinawa Semi-tropical southern island chain reaching out toward Taiwan; formerly the independent Ryukyu Kingdom until it was annexed by Japan in 1879, its traditional customs and architecture are significantly different from the rest of Japan.