noun
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.
気象庁は東京の満開日を3月28日と発表した。
The Japan Meteorological Agency announced that the first day of full bloom in Tokyo is March 28th.
今年の桜の満開日は平年より早い。
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.