The summer solstice is the moment when the Earth's axial tilt is most inclined toward the Sun, resulting in the longest period of daylight and the shortest night of the year. In the Northern Hemisphere, this occurs around June 20 or 21, while in the Southern Hemisphere it falls around December 21 or 22. The word "solstice" comes from the Latin "sol" (sun) and "sistere" (to stand still), because the Sun appears to pause at its highest point in the sky before reversing direction.

The exact date and time of the summer solstice shifts slightly each year because the Earth's orbital period is not exactly 365 days. The Gregorian calendar compensates for this with leap years, but the solstice can still fall on June 20 or 21 depending on the year and time zone. In rare cases, it can even occur on June 22, though this has not happened since 1975 and will not happen again until 2203.

During the summer solstice, locations near the Arctic Circle experience the "midnight sun," where the Sun does not fully set. In Reykjavik, Iceland, the Sun is above the horizon for over 21 hours on the solstice. At the North Pole, the Sun does not set at all from roughly March to September. Conversely, places closer to the equator notice much less difference in daylight hours between seasons.

The Earth takes approximately 365.2422 days to orbit the Sun, which is why we add a leap day every four years. However, this correction is not perfect. Century years are not leap years unless they are also divisible by 400. This creates a pattern where the solstice gradually shifts earlier in the calendar over the course of each century before jumping later after a leap year correction.

The Earth's axial tilt, currently about 23.44 degrees, is also not constant. It oscillates between roughly 22.1 and 24.5 degrees over a cycle of about 41,000 years, known as obliquity. While this does not change the calendar date of the solstice significantly, it does affect how extreme the seasons are. A greater tilt produces more pronounced seasons with hotter summers and colder winters.

The perihelion, or the point in Earth's orbit when it is closest to the Sun, currently falls around January 3, meaning the Northern Hemisphere's summer actually occurs when Earth is farthest from the Sun. This might seem counterintuitive, but the tilt of Earth's axis has a far greater effect on seasonal temperatures than the relatively small variation in distance from the Sun.

Humans have celebrated the summer solstice for millennia. Stonehenge in England, built around 3000 BC, is aligned so that the Sun rises directly over the Heel Stone on the morning of the solstice, casting a beam of light into the heart of the monument. Every year, thousands gather at Stonehenge to watch the sunrise and celebrate the longest day.

In Scandinavia, Midsummer is one of the most important holidays of the year, rivaling Christmas in significance. In Sweden, people raise a maypole decorated with greenery and flowers, dance traditional folk dances, and feast on pickled herring, fresh strawberries, and potatoes. The celebration typically takes place on the Friday closest to June 24. In Finland, Midsummer (Juhannus) is marked with bonfires on lakeshores and a national holiday.

In ancient Egypt, the summer solstice roughly coincided with the rising of the star Sirius, which heralded the annual flooding of the Nile and the start of the agricultural season. The Egyptian New Year was effectively tied to this event. In China, the summer solstice festival (Xiazhi) has been celebrated since at least the Song Dynasty, with traditions including eating cold noodles and giving colored fans as gifts to ward off the summer heat.