The winter solstice, hiemal solstice or hibernal solstice, also known as midwinter, occurs when one of the Earth's poles has its maximum tilt away from the Sun. It happens twice yearly, once in each hemisphere (Northern and Southern). For that hemisphere, the winter solstice is the day with the shortest period of daylight and longest night of the year, when the Sun is at its lowest daily maximum elevation in the sky. At the pole, there is continuous darkness or twilight around the winter solstice. Its opposite is the summer solstice.
The winter solstice occurs during the hemisphere's winter. In the Northern Hemisphere, this is the December solstice (usually 21 or 22 December) and in the Southern Hemisphere, this is the June solstice (usually 20 or 21 June). Although the winter solstice itself lasts only a moment, the term sometimes refers to the day on which it occurs. Other names are "midwinter", the "extreme of winter" (Dongzhi), or the "shortest day". Traditionally, in many temperate regions, the winter solstice is seen as the middle of winter, but today in some countries and calendars, it is seen as the beginning of winter. In meteorology, winter is reckoned as beginning about three weeks before the winter solstice.
Since prehistory, the winter solstice has been seen as a significant time of year in many cultures, and has been marked by festivals and rituals. It marked the symbolic death and rebirth of the Sun. The seasonal significance of the winter solstice is in the reversal of the gradual lengthening of nights and shortening of days.
Seasonal lag is the term relating the lag shift between the coldest winter weather and the winter solstice. As latitude increases, midwinter correlates more closely with the winter solstice.