That's a tough one to plan for. The cherry blossom starts in the south and migrates north over at least several weeks, so it depends on WHERE in Japan you are planning to visit. It also depends on how warm it is that particular year as to when it really starts.
Roughly: sometime in April ... usually. Problem is, the cherry blossom is famous because it is both spectacular and incredibly ephemeral. I've been in and around Kyoto some years ago in April just as it arrived and it was almost all gone after 4 days.
IF you are fortunate enough to get it just right there is little that can prepare you for just how spectacular that particular city can be during those 4 days.
sm about 8 years ago
http://www.jnto.go.jp/eng/indepth/seasonal/sakura2010/ see this - everywhere is different !!! about 8 years ago