The Winter Solstice marks the official first day of winter and officially began at 503 pm CST 12/21/14.
The winter solstice occurs at the instant when the Sun's position in the sky is at its greatest angular distance on the other side of the equatorial plane from the observers' hemisphere. Depending on the shift of the calendar, the event of the winter solstice occurs some time between December 20 and December 23 each year in the northern hemisphere, and between June 20 and June 23 in the southern hemisphere, during either the shortest day or the longest night of the year, which is not to be confused with the darkest day or night or the day with the earliest sunset or latest sunrise.
The seasonal significance of the winter solstice is in the reversal of the gradually lengthening nights and shortening days. How cultures interpret this is varied, since it is sometimes said to astronomically mark either the beginning or middle of a hemisphere's winter. Winter is a subjective term, so there is no scientifically established beginning or middle of winter but the winter solstice itself is clearly calculated to within a second. Though the winter solstice lasts an instant, the term is also used to refer to the full 24-hour period of the day on which it occurs.
The Winter season runs from December through February while the period from the winter solstice to the spring equinox runs from Dec 21 until March 20.