Sakura, cherry blossom is very important tree in Japan. May be the most important one. Due to Japanese school season ends in March and start in April, sakura is a symbol of this season.
Then, most popular season for picnic is during sakura season. People sit down on plastic sheets and eat and drink foods and beverage which they brought there. Due to it is legal to drink alcohol outdoor, most of the people are drinking beer and other alcohol beverage too.
Sakura season depends on the climate and weather each year and it can be earlier or later than the previous year. Therefore it is very hard for foreign tourists to determine their holidays to Japan. However, sakura can be seen at normal parks or may be in normal streets, which means it is not hard to find sakura trees during your stay in Japan if you arrive in the beginning of April.