The days start getting longer every year after the winter solstice, which falls on the first day of winter, the shortest day of the year. The first day of winter is typically December 21 or 22 in the Northern Hemisphere.
Because the Earth's axis of rotation sits at an angle to its orbital plane, for half the year the Northern Hemisphere is inclined toward the sun, accounting for longer periods of daylight. For the other half of the year, the Southern Hemisphere experiences longer periods of daylight. During the course of the year, the Earth slowly tilts back and forth between the two extremes, which are marked by the winter and summer solstices. In the Southern Hemisphere, the seasons are reversed, and the winter solstice occurs in June.