The milder the climate, the earlier the blossoms open.
On Japan’s southern, subtropical islands of Okinawa, cherry blossoms open as early as January, while on the northern island of Hokkaido, they bloom as late as May. In most major cities in between, including Tokyo, Kyoto and Osaka, the cherry blossom season typically takes place in early April.
Furthermore, the blooming time of cherry trees differs from year to year depending on the weather. If the weather during the months and weeks preceding the cherry blossom season is mild, blossoms will open early. If it is cold, blossoms will open later. From year to year, the start of the blooming season can vary by as much as two weeks.
The cherry blossom season is relatively short. Full bloom (mankai) is usually reached within about one week after the opening of the first blossoms (kaika). Another week later, the blooming peak is over and the blossoms are falling from the trees. Strong wind and rain can cut the blooming season even shorter.
More information about the Cherry blossom in Japan is available here.
All about the Cherry Blossom Festival in Washington DC.