Probably the best time to visit Japan, as of course, you have cherry blossom to enjoy with mild weather.
Spring「春」in Japan is usually between March and May. Temperatures are pleasant and sunny, but sweaters and jackets should be prepared for chilly evenings. Depending on weather and temperatures, late February to early March, pink and white plum trees start blossoming at temples, shrines, and public parks.
In mid March to early April is the cherry-blossom time which sweeps up the country from south to north. The seasonal spectacle is celebrated with hanami (cherry blossom viewing) parties under stunning “sakura” trees.
“Golden Week”, the concentration of national holidays at the end of April and the first week of May is the busiest travel season, and best avoided.