Japan is a country with country with a big difference between each season. If you come in August it will be terribly hot, if you come in winter it will be COLD. The best time to come is in spring or the begining of autumn. From the the 1st of March until the 25th of April, or between the 10th of May until the 1st of June are the best timeframes. The last week of April and the first of May are a really bad time to come here because it’s the Golden Week, when all Japanese have holidays and travel all around Japan. From the second week of June until the third of July is the rainy season, that’s not good neither. October and november are probably also a good time, but it is posible that a typhoon would spoil some days of your holiday time. If you don’t mind about hot or cold, February and August are two month when the probability of precitiations is really low. Conclusion: don’t come in the rainy season or during Japanese National Holidays (Golden Weeks and the end/beginning) of the year.