height of summer; midsummer heat
A somewhat formal or literary term for the hottest part of summer. In everyday speech, 真夏 (まなつ) or 盛夏 (せいか) are more common.
In this height of summer, I trust you are all doing well.
盛暑の折、どうぞご自愛ください。
Please take care of yourself during this midsummer heat.