The summer solstice, or midsummer, occurs when a planet’s rotational axis, or geographical pole on either its Northern or its Southern Hemisphere, is most greatly inclined toward the star that it orbits. In Earth’s case, that star is the Sun.
The summer solstice occurs during the hemisphere’s summer. This is the June solstice in the Northern Hemisphere and the December solstice in the Southern Hemisphere. Depending on the shift of the calendar, the summer solstice occurs some time between June 20 and June 22 in the Northern Hemisphere and between December 20 and December 23 each year in the Southern Hemisphere. The same dates in the opposite hemisphere are referred to as the winter solstice.
As seen from a geographic pole, the Sun reaches its highest altitude of the year on the summer solstice. The colloquial term “midsummer” refers to the day on which the solstice occurs. The summer solstice day has the longest period of daylight, except in the polar regions, where daytime remains continuous for 24 hours every day during a period ranging from a few days to six months around the summer solstice.