When the Earth's axis reaches its peak of its tilt, positioning the northern half most directly towards the sun, the Northern Hemisphere experiences its summer solstice. This astronomical event, usually on June 20 or 21, signifies the end of spring and the beginning of summer. For all locations north of the equator, it brings the longest day of the year.
This year, the solstice occurs on June 21 at roughly 3:42am GMT. Simultaneously, the North Pole enters six months of daylight, while the South Pole enters six months of darkness.