A bright "Christmas star" will light up the nighttime sky starting on December 21 when the giant planets Jupiter and Saturn align. advertisement According to GMA’s resident meteorologist Nathaniel Cruz, the rare phenomenon only occurs every 800 years. The “Christmas star” happens when Jupiter and Saturn align, giving the impression of a double planet when viewed from Earth. Observers hoping to catch the alignment of the two planets in the sky can do so after the sunset on December 21. Experts say binoculars or telescopes may help to see the phenomenon.