Today is the Summer Solstice in the northern hemisphere, the day of the year when day is longest and night is shortest. The opposite is the 21st December when the day is shortest and the night the longest. In the southern hemisphere, the opposite is true. The reason for this is that the Earth is tilted at an angle of roughly 23%, caused when the Earth was young and a meteor or planetoid smashed into the Earth. It is not the only planet that is tilted, Uranus is also titled but that tilt is a little more extreme. Uranus rotates on its sides.
The term Solstice comes from Latin where Sol means Sun and Sistere means to stand still. During the ancient times at Stonehenge, Wiltshire, United Kingdom, celebrations and ceremonies would be held. People still congregate there, some who call themselves druids go for religious reasons and others for fun. Rumours and speculations include that during the Solstice, ancient Celts practised human sacrifice to appease the gods.
The Summer Solstice marks the official start of summer until the Autumn/Fall Equinox on the 21st of September. The Equinox is when the length of day is the same as the length of night. On the 21st March, the opposite Equinox occurs in that Spring begins and then days becomes longer than the nights.