For people who live north of the equator, the shortest day of the year falls around Dec. 22. This day is called the winter solstice. (The longest day of the year is called the summer solstice, and falls around June 21.)
But the winter solstice falls at the beginning of winter, not in the colder months that follow, and therefore the shortest day of the year is almost never the coldest.
The weather is warmer on the shortest day of the year than it is in midwinter because the earth’s atmosphere holds in a lot of the summer and autumn heat. The atmosphere gradually loses this heat until sometime in January.
The coldest days come after the heat has been lost. That’s why it’s usually colder in February than it is in December, even though the days in February are longer.