A solstice is the moment in the year when the sun reaches its furthest point north or south, and so the day is either the longest or the shortest of the whole year. There are two every year. The summer solstice is around 21 June in the northern half of the world, and that is the longest day. The winter solstice is around 21 or 22 December, the shortest day. South of the equator it is the other way round — June is their winter solstice. Strictly speaking a solstice is a moment, not a day: astronomers can tell you the exact hour and minute. In ordinary talk, though, people use it for the whole day: 'we're going to the beach on the solstice'. The word is a bit formal or scientific, but it also has a warm, old feeling to it, because people have been marking these days with festivals for thousands of years — that is why you meet it in newspapers about Stonehenge, and in the names of parties and music festivals. Note that the solstice is about daylight length, not about temperature: the longest day is not the hottest day of the year, because the land and sea take weeks to warm up. This is why English speakers sometimes call the June solstice 'midsummer' even though it is the beginning of summer — the naming is older than the science. The other pair of dates, when day and night are equal, are the equinoxes, in March and September.