midsummer day and midsummer night

midsummer day and midsummer night names given to the feast of the nativity of St. John the Baptist (June 24) and the preceding night (St. John's Eve, June 23). Because midsummer is about the time of the solstice, it has been associated with solar ceremonies since long before Christianity. Relics of such ceremonies are the bright bonfires and the merrymaking of midsummer night. Formerly it was considered the one night of the year when supernatural beings were about. The importance of this night to love and lovers is undoubtedly a survival of fertility rites. Shakespeare and Mendelssohn are among the masters who have used the festival as a subject for a major work.

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.