Midsummer Eve

1. June 23, the day before Midsummer Day, celebrated in Europe, Latin America, and Scandinavian communities in the United States by merrymaking.
2. June 23, the day before Midsummer Day, celebrated in many Christian churches by feasting. Also called Saint John's Eve.

The noun has one meaning:

Meaning #1: the night before Midsummer Day
  Synonyms: Midsummer Night, St John's Eve, St John's Night, June 23