Winter solstice

The annual moment when one hemisphere is tilted farthest from the Sun, producing its shortest day and longest night. It occurs once each year in each hemisphere.

Earth's axial tilt: Earth’s rotational axis is inclined about 23.4 degrees relative to the perpendicular to its orbital plane. The tilt points the winter hemisphere away from the Sun at the solstice.
Day length: The interval between sunrise and sunset at a particular location and date. The winter solstice brings the shortest daylight interval of the year in its hemisphere.
Tropical year: The time Earth takes to complete a cycle of seasons, averaging about 365.2422 days. The tropical year is measured from one seasonal point, such as a solstice, to the next.
Summer solstice: The annual moment when one hemisphere is tilted most toward the Sun, producing its longest day and shortest night. It is the opposite seasonal extreme, occurring about six months later in the same hemisphere.
Yule: A midwinter festival historically observed by Germanic peoples and later incorporated into Christian seasonal traditions. Yule is associated with midwinter observances near the winter solstice.
Solstice: Either of the two annual moments when the Sun reaches its greatest apparent distance north or south of the celestial equator. The winter solstice is one of the two seasonal solstices.
Hemisphere: One half of a sphere, especially either half of Earth divided by the equator. The Northern and Southern Hemispheres experience winter solstices six months apart.
Gregorian calendar: The civil calendar introduced in 1582 to keep calendar dates aligned with the seasons. Its leap-year rules keep solstice dates from drifting substantially through the calendar.
Equinox: Either of the two annual moments when the Sun crosses the celestial equator and day and night are nearly equal. Unlike a solstice, an equinox occurs when neither hemisphere is tilted most toward the Sun.
Saturnalia: An ancient Roman festival honoring Saturn, held in December with feasting and gift-giving. Its timing near the solstice links Roman winter celebrations with the season’s turning point.

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