The moment the Sun reaches its southernmost point in the sky marks the December solstice, the official beginning of winter in the Northern Hemisphere (where it is called the winter solstice) and a time of great celebration in many northern cultures.
The seasons' starting times are governed by Earth's motion around the Sun — or equivalently, from our point of view, the Sun's annual motion in Earth's sky. The start of winter (for the Northern Hemisphere) is defined as the moment when the Sun hovers over Earth's Tropic of Capricorn (the line of latitude 23½° south of the equator) before heading north — a moment called, by Northerners, the winter solstice.
The Sun appears to move north and south in our sky during the year because of what some might consider an awkward misalignment of our planet. Earth's axis is tilted with respect to our orbit around the Sun. So when we're on one side of our orbit, the Northern Hemisphere is tipped sunward and gets heated by more direct solar rays, making summer. When we're on the opposite side of our orbit, the Northern Hemisphere is tipped away from the Sun. The solar rays come in at a lower slant to this part of the world and heat the ground less, making winter.
The effect is opposite for inhabitants of the Southern Hemisphere; for them the December solstice signals the beginning of summer, while winter starts at the June solstice.
For a skywatcher on Earth (at north temperate latitudes), the effect is to make the Sun appear to move higher in the midday sky each day from December to June, and back down again from June to December. A solstice comes when the Sun is at the upper or lower end of its journey; an equinox comes when the Sun is halfway through each journey.
The word solstice comes from the Latin solstitium — sol meaning "sun" and -stitium "stoppage." The winter solstice marks the shortest day and longest night of the year. From now on, the days begin to grow longer and the nights shorter. In ancient cultures, the winter solstice was an auspicious moment. It meant the end of declining hours of sunlight and provided a sense of renewal as the Sun began its daily climb higher in the sky.
Winter-solstice celebrations could well be the world's oldest holidays. There are more known rituals associated with this solstice than for any other time of the year. Prior to the Christian era, Romans called this day Dies Natalis Invicti Solis, the Birthday of the Unconquered Sun. Earlier in Rome it was the time of Saturnalia, a notoriously wild holiday. In 46 BC the winter solstice fell around December 25th. Despite calendar reforms, these celebrations — and the observance of Christmas by early Christians — remained locked to the 25th.