Updated: Dec 5, 2020
The winter solstice is the first day of winter & the shortest day of the year in Earth's northern hemisphere...and the longest day of the year in the Southern hemisphere. It occurs annually between December 20th and December 23rd, depending on if it is a leap year among other factors. The Gregorian calendar that we use daily doesn't line up with astronomical cycles perfectly.
Earth's seasons, equinoxes, and solstices are the result of Earth's revolution/orbit around the Sun. In December the North Pole is pointing away from the Sun so the Northern hemisphere gets less light & experiences winter conditions. When the short days of winter seem endless, consider that there is no sunrise at the North Pole until the spring equinox in March.
The seasons cycle through during Earth's annual trek around the Sun, and the amount of sunlight a region receives is dependent on its latitude and the date. The Earth's axial tilt remains relatively constant, because it takes 21,000 years for the planetary tilt to change by less than 2.5 degrees, so we don't see any difference during our lifetime. Instead the seasons are the result of Earth's location along its year long orbital path.
It just so happens that we are actually closer to the sun in December than we are in June, so the seasonal weather in the Northern Hemisphere is less extreme than it is in the Southern Hemisphere.
We celebrate winter solstice as the shortest day of the year, knowing that each day that follows will get a little brighter and eventually the longer days will bring warmer weather.