From the Japan Meteorological Agency (JMA).
The Japan Meteorological Agency (JMA) watches plants at its weather stations across Japan, using the same rules everywhere. For each plant it chooses one tree to watch, called the sample tree.
For cherry trees, which have many flowers, flowering is the first day when 5 or 6 flowers are open on the sample tree. For other plants it is 2 or 3 flowers.
Full bloom is the first day when about 80% of the buds are open. The JMA says it is hard to tell if every single flower on a big tree is open, so it uses “about 80%”, when people feel the tree is in full bloom.
The main kind the JMA watches is Somei-yoshino. It began at the end of the Edo period, and it is a cross between Edo-higan and Oshima-zakura.
It is planted from Kyushu to around the Ishikari Plain in Hokkaido. Where it does not grow, the JMA watches hikan-zakura or ezo-yama-zakura instead.
In a normal year, hikan-zakura begins to flower in the middle of January in Okinawa and Amami.
Somei-yoshino flowers in late March along a line from Kyushu to southern Kanto, and by April 10 the line reaches Hokuriku, northern Kanto and southern Tohoku. It then moves north and reaches northern and eastern Hokkaido in the middle of May.
In 2026, cherry trees in Naha (hikan-zakura) flowered on January 18. In Tokyo, they flowered on March 19, 5 days earlier than normal. In Sapporo, they flowered on April 18, 13 days earlier than normal.
There is no single right answer. Talking it over with someone at home is a good way to learn.