It is a Japanese custom to see and enjoy the beauty of blooming flowers (mostly sakura) during spring time. Normally done during mid March until mid April, when sakura flowers bloom all over Japan.
In Hanami, Japanese do some kind of outdoor party; they go to koen (公園 ; park) or other public places where there are sakura trees, bringing their mat, food and drinks, just like picnic.
Last spring I had this Hanami with my colleagues in Tsutsuji Park Iwata City near our dormitory. Pictures below! :D