The summer solstice (or estival solstice), also known as 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.
It is the longest day of the year. Solstices are opposite on either side of the equator, so the summer solstice in the Northern Hemisphere is the winter solstice in the Southern Hemisphere, and vice versa.
In the USA and some other areas in the Northern Hemisphere, the summer solstice marks the first day of summer. However, the official date for the first day of summer varies depending on the country's climate.
One of the world's oldest evidence of the summer solstice's importance in culture is Stonehenge in England, a megalithic structure which clearly marks the moment of the June solstice.
In the Southern Hemisphere, where the June solstice is known as the shortest day of the year, it marks the first day of astronomical winter, but the middle of winter in meteorological terms.
Each solstice marks a “turning” of the year. Even as this northern summer begins with the solstice, throughout the world the solstice also represents a “turning” of the year. To many cultures, the solstice can mean a limit or a culmination of something. From around the world, the sun is now setting and rising as far north as it ever does. The solstice marks when the sun reaches its northernmost point for the year. After the June solstice, the sun will begin its subtle shift southward on the sky’s dome again.