Cherry blossom is three weeks of it. Flip through the other forty-nine — and watch what's in bloom move up and down the country.
The calendar is the way in. Underneath it sits the ordinary thing — search, filter by month or flower, and open any of the 412 spots.

Photo Hyo-tan (CC BY-SA 4.0) — via Wikimedia Commons
The great greenhouse of Tokyo's largest botanical garden gained a begonia room in its 2016 renewal, filled with large-flowered tuberous begonias and hanging baskets, so begonias are in flower here whatever the season. Adults 500 yen.