It may seem like winter already but the official start of the winter solstice for the Northern Hemisphere begins Saturday, December 21st and runs until Wednesday, March 19th. Winter begins when the sun reaches its most southerly declination of -23.5 degrees.

Easier said, the North Pole is tilted 23.5 degrees. The December 21st or 22nd winter solstices occur more frequently than the December 20th and 23rd solstice. The last December 23rd solstice was in 1903 and the December 23rd has occurred very rarely with the next one not due until December 23rd of 2080.

The varying dates are due to the calendar which has 365 days in a common year and 366 in a leap year. The sun returning to the same position as seen by Earth is different than a calendar year.

Timeanddate.com