Jupiter and Saturn will align on the winter solstice for a phenomena not seen in 800 years.
The two planets will align close enough on December 21st to become what is known as the "Christmas Star." The last time this celestial event called the "great conjunction" happened was on March 4, 1226. Not only that, but some astrologers believe that this event was the "Star of Bethlehem" seen by the three wise men in the Nativity story.
Jupiter and Saturn meet every 20 years, but this time they will only be a fifth of a full-moon diameter, or .1 degrees, apart. That hasn't happened since the Middle Ages.
The next time the conjunction will happen this close will be in March of 2080.