Spring is a good time to visit Japan, cool weather and clear blue skies. For the sakuras, the best is to go during the mid-bloom season, typically around 2-3 days after the first blooms, cos then the trees will be full of them. If you want to take good pictures, go early to avoid the crowd and best is to avoid the famous places like Ueno Park etc etc. Kyoto has a lot of temples which are less well-known but still have beautiful sakura blooms. Some people like to take pictures of the sakuras near the castles cos they will usually be lit at night.
