The distinct seasons not only charactelize Japan, but also provide many beautiful landsacapes and tourists attractions. Even the same place would give you totally different impressions depending on when you visit there.
Most people associate spring of Japan with Sakura (cherryblossoms). You can enjoy full bloom of Sakura from late March to early May, but peak periods vary significantly among regions. For example, the peak period in Tokyo usually from late March to early April, while those in Tohoku and Hokkaido usually from late April to early May. Moreover, full bloom tipically lasts only for a few days. Thus, it is very important to check bloom forecasts and plann a schedule beforehand. One of the forecasts is the following: https://www.jnto.go.jp/sakura/eng/index.php
Flowers other than Sakura also bloom in spring and attracts many people. Kawazu-zakura have become famous as some of the earliest-blooming cherry blossoms. Compared to normal Sakura (Somei-yoshino), Kawazu-zakura are charactelized by its large petals and the bright pink color. Kawazu-zakura’s peak season is usually from late February to early March, and they tend to have longer peak periods. Kawazu, in Shizuoka prefecture, is the most famous place to see Kawazu-zakura. In addition, Nemophila and Nanohana add colors to spring of Japan.
Like many countries, summer is the peak season of tourism in Japan. Especially, Okinawa has many beatiful beaches and is famous for coral leaves around the island. Around Tokyo area, Izu peninsula (Shizuoka prefecture) and Boso peninsula (Chiba Prefecture) are known for beautiful beaches and attract many divers from Tokyo. On mountain side, summer is the best season for hiking, as higher mountains such as Mt. Fuji and the Japanese Alps are only open for summer season.
From June to mid July, most of areas in Japan (except Hokkaido) have the rainy season. The rainy season is usually a lower period for tourists, but Ajisai (Hydrangea macrophylla) bloom during the period and are associated with the rainy season.
In many regions, summer is also a season of festivals and fireworks. You could have special experiences if you visit these festivals.
You can enjoy a variety of autumn foliage in Japan. Autumn foliage starts from Hokkaido and high mountains in late September. In Tokyo and Kyoto, peaks usually come around late November. Major places to see fall foliage are located in both mountain side and cities. In Kyoto, many temples and shrines attracts tourists with their beautiful autumn leaves.
Autumn is also a season of harvest. Many vegetables and fishes are in season and you can taste seasonal dishes, such as Matsutake mashrooms and sauries.
In many countries, winter is considered to be off-season, but it is not necessarily the case for Japan. First, in northern and mountain areas, you can see a lot of snow. In particular, Hokkaido, Tohoku and Hokuriku are famous for their ski resorts, which attracts many foreign skiers.
Moreover, in Pacific side, winter has more sunny days. Therefore, winter is the season for seeing night views.