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 (Northern and Southern). For that hemisphere, the summer solstice is the day with the longest period of daylight and the 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. Although the summer solstice is the longest day of the year for that hemisphere, the dates of the earliest sunrise and latest sunset vary by a few days. This is because Earth orbits the Sun in an ellipse, and its orbital speed varies slightly during the year.

Source: Wikipedia