In 2013, the shortest day of the year will occur on December 21. Leading up to that day, the days are going to get shorter. Once that day passes, the days will start getting longer again. The shortest day is also referred to as winter solstice. December 21 will also be the shortest day of the year in 2014, 2016, 2017, 2018, and 2020. In 2015 and 2019, the shortest day will occur on December 22.