In Great Britain from the 13th century Midsummer was celebrated on Midsummer Eve (St. John's Eve, June 23) and St. Peter's Eve (June 28) with the lighting of bonfires, feasting and merrymaking. This year, 2009, the exact time of the summer solstice is 0545 on Sunday 21st of June

Midsummer's Eve is June 23rd- also called Saint John's Day.
The celebration of midsummer's Eve was from ancient times linked to the summer solstice. People believed that mid-summer plants had miraculous and healing power; so the plants were picked on this night. Bonfires were lit to protect against evil spirits;which were believed to roam freely when the sun turned southward again.

Eve Leoff has written:

'William Shakespeare's A midsummer night's dream'

Midsummer's Eve is special to lovers because of a story in Estonian folklore. It is said that if lovers go into the forest on Midsummer's Eve night and find a special flower their love will be blessed.

No. "Night", yes, but what does John have to do with it?