One of the most important things to know about Japan is that they have the biggest internal tourism in the World: that means that on high-season (or school vacation, or specific holidays), Japanese will travel around and book the hotels as they go. That and the fact they have a current deficit in Hotel space, the general rule of thumb is to book your stay in advance.
The image bellow shows you the Japanese Tourist-season calendar, with high-seasons in orange.
The following is a non-exhaustive list of the Touristic Seasons, as well Holidays that might get you locked out of the best cost/benefit hotels.
- The spring - or Hanami (flower viewing, more specifically the Cherry Blossoms) pack the country to the maximum in the final weeks of March and early April. If you arrive a couple of weeks earlier, you will catch the plum blossoms.
- The Golden Week - Several Holidays packed together, usually causing a whole week off work or school: April 29th (Showa Day), May 3rd (Constitution Day), May 4th (Greenery Day) and May 5th (Children's Day) will cause a whole week of high season in early May.
- The Autumn - or Koyo (Autumn leafs or Autumn colors) are a less crowded high season not because it is less impressive, but because it spreads through more days: it takes the whole of October, November and the first week of December. Choose a time and place to appreciate best following this Japan-Guide map ↗
These are not as crowded, but might take you by surprise on the day(s) they happen. Note most of these happen in particular cities.
- New Year - December 31st and January 1st are really Holy and most commerce comes to a halt. Shrines and Temples get packed and a lot of parks and gardens receive increase visitors.
- Sanja Matsuri - Remarkable festival around Asakusa Shrine in Tokyo during the 3rd weekend of May
- Gion Matsuri - Important festival in Kyoto, around Yasaka Shrine on (and around) July 17th
- Tenjin Matsuri - An interesting river festival around Tenmangu Shrine in Osaka in July 25th
- Halloween - While not a traditional Japanese Holiday, it has been picking up momentum lately and was quite remarkable in 2016, so one should expect it to be more and more present on October 31st specially around areas crowded by young adults.
- Chichibu Yomatsuri - A Night festival in Saitama in the evening of December 3rd
It is worth noting two periods: The rainy season (June to August), when you should be ready for indoors days, and the Typhoon season, which peaks from August to September, when you might get delayed flights due to Typhoons. Read more about it here
The winter and summer are the low season. The winter because of the cold (despite Japan having its offering of Ski resorts) and the summer mostly because it is the rainy season, with regions raining for over 50% of the days. It rains in most of the country in June and July, but some areas have higher than average rainfall up to September.