Winter is one of the four seasons, coming after autumn and before spring. In the northern hemisphere, winter occurs between the December solstice and the March equinox. In the southern hemisphere, it occurs between the June solstice and the September equinox.
Winter is marked by decreased temperatures, often the lowest of the year. This is due to the tilt of the Earth's axis causing sunlight to be reduced in a given hemisphere as the Earth orbits the Sun. Christmas and Saint Valentine's Day are examples of holidays that are celebrated during winter, and Saint Patrick's Day is celebrated at the near end of winter.