Definition: Winter officially begins on the shortest day of the year December 21 (or December 22 in some years) the day of the solstice.
Solstice means the sun stands still. The winter solstice is the day when the midday sun is at its lowest point above the horizon.
Often celebrated with parties and winter customs.
Celebrated on December 21 [or 22] (source: kidsturncentral.com)