Hanami (flower viewing) is the Japanese traditional custom of enjoying the transient beauty of flowers; flowers (hana) are in this case almost always referring to those of the cherry (sakura) or, less frequently plum(ume) trees. Cherry trees generally bloom from the end of March to early May. The blossom forecast (桜前線, sakura-zensen) is announced each year by the Japanese weather bureau and is watched carefully by those planning hanami as the blossoms only last a week or two.
(Thankfully the current lockdown does not affect nature and has maybe given us more time to appreciate its beauty. On my daily walks I have been enjoying the beautiful blossom which is now just starting to fall. Ed.)