You've heard of meteorological winter and "astronomical" winter. But what's the difference?
Astronomical winter is based on the relationship of the sun's direct rays to the Earth. As you probably recall, the earth's axis is tilted at around 23.5 degrees, which means that throughout the year and throughout our journey around the sun the angle of the sun will vary through the year. That's why sunrises and sunsets change through the year, with winter featuring the shortest days and summer the longest. Tomorrow, the sun's most direct rays will touch the Tropic of Capricorn in the Southern Hemisphere and then we wobble back towards the Tropic of Cancer in six month's time. From here on out, our day length will slowly increase while it decreases south of the equator. Basically, astronomical winter is the three month division of time from when the sun's direct rays reach the Tropic of Capricorn and return to the equator.
From a meteorological and climate data keeping perspective, it's a matter of data division. December, January, and February are typically the three coldest months in climatic record in the Northern Hemisphere and since many climate records are kept on a monthly basis, it's easier to track data by season using the months instead of the days that winter is made up of. Given that winter some years starts on the 21st, some years on the 22nd, data tracking and consistency from year to year given the starting date's change leads to using a more consistent data division...that's why the December-February period of time have been used to define "meteorological" winter.
In any case, it's still the coldest weeks of the year forthcoming...and after tomorrow, there's no argument about what season it is as winter officially starts up.
More: Tropic of Cancer (including a cool graphic that shows how the position "changes" over time).