first day of full bloom (cherry blossoms)
Specifically refers to the first day when at least 80% of Somei Yoshino cherry blossoms have opened in a given region or location, as used in weather forecasts and sakura reports.
The Japan Meteorological Agency announced that the first day of full bloom in Tokyo is March 28th.
This year's first day of full bloom for cherry blossoms is earlier than average.
満開 is the state of being in full bloom, whereas 満開日 specifies the exact day that state is first reached.
Compound of 満開 (full bloom) + 日 (day). The term is used in meteorological contexts to mark the peak of cherry blossom season.