What is it? The solstice occurs twice a year when the Earth's axis tilts furthest towards or away from the Sun.
The name solstice is taken from the Latin "sol", meaning sun, and "sistere", meaning to stand still. The term also refers to the whole day on which this passage of the Sun occurs.
In some parts of the world the solstice begins the seasons, while in the UK they are considered to be centre points of the year, occurring within days of midsummer and midwinter.
The summer solstice is on 21 June and is the longest day of the year for the northern hemisphere, with about 17 hours of daylight in the UK.
Where will it be celebrated? English Heritage will be bracing itself for the annual influx of druids and pagans to Stonehenge in Wiltshire.
The stone circle is the big draw for people who mark the summer solstice and is one of the few times of year when visitors are allowed to walk among the stones. Thousands are expected to flock there on Wednesday night to see in the dawn.
It is thought Stonehenge, built from bluestone, sarsen and Welsh sandstone, dates from 3100BC and took 30 million man-hours to construct.
What will they do when they get there? Stonehenge is a pagan symbol and druids, a pagan order claiming to date back to Celtic Britain, will lead the celebrations. For the less spiritual, it is an opportunity to indulge in recreational substance abuse, while others see it as a warm-up act for the Glastonbury Festival the next day.