For those north of the equator, the shortest day of the year occurs each year more or less on December twenty-first. It may be a day or two earlier some years or a day or two later other years, but it's always on or near the twenty-first. Some people would conclude from this that the coldest days of the year should occur right around this time, but due to a phenomenon known as seasonal lag, the coldest days of the year usually occur during early January in the Northern Hemisphere. Of course, in the Southern Hemisphere everything is off by six months with Winter Solstice happening on or around June twenty-first and so while people living north of the equator are experiencing Winter Solstice, those living on the other side of the equator are experiencing Summer Solstice, which is the longest day of the year. Half way between the two solstices are the equinoxes which occur on or around March twenty-first and September twenty-first. The four seasons begin and end on the solstices and the equinoxes with those north and south experiencing opposite seasons throughout the year.