When is the midpoint of winter?

Every year on Feb. 2, the U.S. mainstream media focuses its attention on the little community of Punxsutawney, Pennsylvania, to see if a groundhog named Phil, emerging from his burrow on Gobblers Knob, will either see his shadow (meaning winter will persist for six more weeks) or does not see his shadow (foretelling an early spring). 

On many calendars, this is also known as Candlemas, the “40th day of Christmas” (that is, of a period which includes Christmas as its first day). The legend of the groundhog likely originated from this axiom about Candlemas: