December 21 is the 355th day of the year in the Gregorian calendar (356th in leap years). Ten days remain until the end of the year. The entry point in the modern civil calendar is governed by the Gregorian calendar, so observances and weekday placements repeat on long cycles.

Overview

This date is widely known for its astronomical and cultural associations. In the Northern Hemisphere it is commonly the day of the December solstice, the moment when one of Earth’s poles tilts farthest from the Sun. That event produces the shortest daytime and longest night of the year in northern latitudes and the opposite effect in the Southern Hemisphere. The exact moment of the solstice can shift between December 20 and December 22 because of leap-year rules and orbital dynamics.

Astronomical significance

The solstice arises from Earth’s axial tilt: as the planet orbits the Sun, the tilt determines where sunlight is most direct. Around December 21 the Sun reaches its most southerly declination, and in tropical astrology the Sun typically moves into Capricorn around this period. Polar regions experience extreme conditions—continuous night (polar night) in the Arctic and continuous daylight (midnight sun) in parts of the Antarctic.

Cultural observances

  • Yule and other winter solstice festivals: pre-Christian and modern celebrations marking the return of light.
  • Dongzhi: a traditional East Asian festival that honors the winter solstice and family gatherings.
  • Many contemporary seasonal traditions and holidays cluster around late December, so December 21 is often part of extended celebrations and civic observances.

Notable facts and events

Various historical events and cultural moments are recorded on December 21. Examples frequently cited include the publication of the first modern-style crossword puzzle in a newspaper (1913) and high-profile incidents such as the 1988 bombing of Pan Am Flight 103 over Lockerbie. The date also entered popular culture because of the 2012 phenomenon, when the end of a cycle in the Maya Long Count calendar was widely discussed in media and public imagination.

In summary, December 21 combines calendar bookkeeping (day number and position in the year), predictable astronomical patterns, and a range of cultural meanings. Its significance varies by latitude, culture and historical context, but it remains one of the most noted dates of the year for both science and tradition.