The period when something (an era, movement, organization, etc.) is at its peak; the heyday or golden age.
あの映画監督は1990年代が最盛期だった。
That film director's heyday was in the 1990s.
企業の最盛期には従業員が千人を超えていた。
At the company's peak it had over a thousand employees.
この観光地は夏が最盛期で、毎年多くの人が訪れる。
This tourist spot's peak season is summer, and many people visit every year.
The best season or peak time for produce (fruit, vegetables, flowers, etc.); the time of best quality or harvest.
このりんごは今が最盛期で、とても甘い。
These apples are at their best now and are very sweet.
菜の花は春に最盛期を迎える。
Rapeseed flowers reach their peak season in spring.
観賞用の花は種類によって最盛期が異なる。
The peak season for ornamental flowers differs depending on the species.