The temperature and weather really depends on the month, the season and also the year (like any country!). Obviously, it also depends on the region, prefecture, city, etc. Find below a comprehensive chart of the average temperatures and weather patterns of overall Japan from Climate Temp Info. Postings of information in °F and graphs are also available.
|Avg. MIN temp (°C)||Avg. MAX temp (°C)||Avg. temp (°C)||Avg. rainfall (mm)||Wet days (>0.1 mm)||Avg. sunlight hrs/day||Relative humidity||Avg. wind speed (beaufort)|
Other sites to check out would be: Japan Guide which shows averages for 6 major cities and also has some interesting info about "When to Travel Japan", and BBC which lists averages for Tokyo and has other useful information as well!