Saint John's Eve, starting at sunset on 23 June, is the eve of the feast day of Saint John the Baptist. This is one of the very few feast days marking a saint's birth, rather than their death. The Gospel of Luke states that John was born six months before Jesus; therefore, the feast of John the Baptist was fixed on 24 June, six months before Christmas. In the Roman calendar, 24 June was the date of the summer solstice, and Saint John's Eve is closely associated with Midsummer festivities in Europe. Traditions are similar to those of May Day and include bonfires, feasting, processions, church services, and gathering wild plants.

The summer solstice or estival solstice occurs when one of Earth's poles has its maximum tilt toward the Sun. It happens twice yearly, once in each hemisphere. For that hemisphere, the summer solstice is the day with the longest period of daylight and shortest night of the year, when the Sun is at its highest position in the sky. At either pole there is continuous daylight at the time of its summer solstice. The opposite event is the winter solstice.