Cherry Blossoms (Sakura) are Japan's unoffical flower. These flowers bloom annually and there are many "Cherry Blossom Festivals" that can be experienced throughout Japan. Depending on where you are in Japan the Cherry Blossoms may bloom as early as January or as late as May. These beautiful flowers do not last very long so you'll want to catch the season at the right time.
Helpful Tip:
Check this
site for an estimated time of the first blossoms.