Because the Earth's axis is tilted, the length of a day changes during the course of a year, with the longest stretch of daylight in midsummer, and the shortest day falling in midwinter.
It's based on Earth's position in relation to the sun and the 21st is when we'll see the winter solstice, the shortest day of the year when astronomical winter begins.
It falls at the midpoint between the winter solstice and the spring or vernal equinox, making it the ideal time to consider when springtime might finally come.
In summer and winter, the fighting continued. The sun burned like fire. The soldiers marched on. The cold winter winds blew snow in their faces. The soldiers marched on.
It is very difficult to obtain, as when the fruit ripens, on a full-moon night around the winter solstice, the fruit explodes as a mechanism to spread its seeds.