Winter is approximately 3 months long — at least by the calendar. Officially, winter begins on the winter solstice and ends on the spring equinox.
In the northern hemisphere, the winter solstice is December 21 or 22. It is also the shortest day of the year, as the Earth is tilted so that the southern hemisphere receives more daylight. In astronomical terms, it means the sun is the farthest away from the celestial equator, which is an imaginary line that runs around the earth above the equator. The same day marks the summer solstice in the southern hemisphere. Alternatively, the spring equinox signals the beginning of spring. It is always March 20 or 21. Therefore, in the northern hemisphere, winter lasts from December 21 or 22 to March 20 or 21, about three months. Nonetheless, depending on location, winter weather can last much longer. For instance, winter weather can run as long as September to May (or all year) in places like Alaska or northern Canada.