21 December
On 21 or 22 December every year, the 'solstice' occurs.
For those who live in the southern hemisphere of Earth, on 21/22 December every year is the summer solstice, the year’s longest day. It represents the day when the Sun, as seen from Earth, is furthest south. The winter solstice, on June 21/22, is the shortest day of the year, when the Sun is seen furthest north.
The situation is opposite in the Earth’s northern hemisphere where December has the shortest day and June the longest.
Solstices and the variation of seasons depend on the fact that the rotation axis of Earth is not 'vertical' with respect to its orbital plane around the Sun. The axis is instead inclined by 23 degrees 27 minutes.
