Midsummer is a celebration of the season of summer, taking place on or near the date of the summer solstice in the Northern Hemisphere, the longest day of the year. The name "midsummer" mainly refers to summer solstice festivals of European origin. These cultures traditionally regard it as the middle of summer, with the season beginning on May Day. Although the summer solstice falls on 20, 21 or 22 June in the Northern Hemisphere, it was traditionally reckoned to fall on 23–24 June in much of Europe. These dates were Christianized as Saint John's Eve and Saint John's Day. It is usually celebrated with outdoor gatherings that include bonfires and feasting.

There is evidence that the summer solstice has been culturally important since the Neolithic era, with many ancient monuments throughout Eurasia and the Americas aligned with sunrise or sunset on the summer solstice.

In the Julian calendar used in the ancient Roman world, the date of the summer solstice was 24 June, and Marcus Terentius Varro wrote in the 1st century BC that the Romans saw this as the middle of summer. In the city of Rome, it was the festival of the goddess Fors Fortuna. People thronged the River Tiber and rowed in boats to the temples of Fortuna; "after undisclosed rituals they rowed back, garlanded and inebriated".

The Julian calendar had a flaw in that the solstices and equinoxes gradually fell on earlier dates. At the First Council of Nicaea (325), the Christian Church set the date of the spring equinox to 21 March on the Julian calendar, for the purpose of calculating Easter. This also brought the date of the summer solstice forward to 20 June.

The name 'midsummer' is attested in Old English as midsumor, which meant the summer solstice. It was seen as the middle of summer in Anglo-Saxon England, with the season beginning in early May. Some Anglo-Saxon calendars place midsummer on 24 June while others place it on 20 June. Saint John's Day on 24 June was called middes sumeres mæssedæg or middesumores mæsse (Midsummer's Mass-day).

In England, 24 June continued to be called Midsummer Day and was one of the quarter days of the English calendar. Elsewhere in northern Europe, midsummer and the solstice were traditionally reckoned as the night of 23–24 June.

Sandra Billington says there is no evidence that the pre-Christian Germanic peoples celebrated the summer solstice. This is directly contradicted by a Midsummer Heathen festival cited in Óláfs saga Tryggvasonar known as miðsumarsblót ("midsummer sacrifice"). In the saga a Christian king is invited to a Heathen festival during Midsummer after agreeing to witness it when proselytising.

The historian Ronald Hutton says that the "lighting of festive fires upon Saint John's Eve is first recorded as a popular custom by Jean Belethus, a theologian at the University of Paris, in the early twelfth century", but is undoubtedly much older. In England, the earliest reference to this custom occurs in the 13th century AD, in the Liber Memorandum of the parish church at Barnwell in the Nene Valley, which stated that parish youth would gather on the day to light fires, sing songs and play games. A Christian monk of Lilleshall Abbey, in the same century, wrote: