Midsummer another term for the Summer Solstice, and the period of time immediately surrounding it.
Midsummer Day 24 June, the feast of the Nativity of St John the Baptist, a quarter day in England, Wales, and Ireland, originally coinciding with the summer solstice and in some countries marked by a summer festival.
midsummer madness foolish or reckless behaviour, considered to be at its height at midsummer, and sometimes attributed to the midsummer moon of the lunar month in which Midsummer Day falls.

mid·sum·mer / ˈmidˈsəmər/ • n. [often as adj.] the middle part of summer: the midsummer heat. ∎ another term for summer solstice.