Refers to the hottest part of summer, typically late July through August. Often used in weather forecasts, seasonal greetings, and formal writing.
In this midsummer season, I hope you are all in good health and spirits.
今年の盛夏は特に暑さが厳しい。
The midsummer heat is especially intense this year.
盛夏 is a formal, literary word for the height of summer, close in meaning to 暑中 but rarely used in greeting phrases.
Compound of 盛 (prosperous, peak) and 夏 (summer), literally 'peak summer'.