Friday, Dec. 21, is the first day of winter, and the shortest day of this year. This astronomical start of the season officially starts at 4:23 p.m., local time. However, by some accounts, winter began on Dec. 1.

Dec. 1 is the meteorological start of the season, based on the 12-month calendar, while Dec. 21 is the astronomical start, based on the position of the sun in relation to the earth.

In 2019, the seasons’ meteorological starting dates will be: March 1, Spring; June 1, Summer; September 1, Fall and December 1, Winter, while their astronomical starting dates will be March 20, June 21, Sept. 23 and Dec. 21.

According to the Old Farmer’s Almanac, the astronomical start of each season is marked by either a solstice (for winter and summer) or an equinox (for spring and autumn). A solstice is when the sun reaches the most southerly or northerly point in the sky, while an equinox is when the sun passes over earth’s equator. Because of leap years, the dates of the equinoxes and solstices can shift by a day or two over time, causing the start dates of the seasons to shift, too. In contrast, the meteorological start of a season is based on the annual temperature cycle and the 12-month calendar.

The winter solstice is the day with the least amount of sunlight. After that day, the amount of daylight begins to increase.

Coincidentally, December’s full moon will also appear on the night of winter solstice this year. Native Americans had names for each month’s full moon. December’s full moon is called the Full Cold Moon or the Long Nights Moon. This year the Full Cold Moon will appear on Friday, Dec. 21, though it will not be at its absolute peak until Saturday.