The winter solstice doesn't always occur on December 21. Sometimes it comes on December 22, as will happen in 2015. The hour of day on which the winter is said to set in also fluctuates. Last year, the solstice had occurred at 11:12 a.m. ET and the next year, it occurs at 11:03 p.m.
This year, today the winter solstice is at 12:11 p.m. ET, when the North Pole is tipped about 23 degrees away from the sun. Thus, the winter solstice is the shortest day and the longest night of the year in the Northern Hemisphere.
From religious and cultural point of view, the interpretation of the shortest day has varied from culture to culture with prayers, festivals, gatherings, rituals or other social celebrations on this day.