What is a Christmas star? German astronomer Johannes Kepler theorized that that the “star of Bethlehem” in the story of the “three wise men” was a rare triple conjunction of Jupiter, Saturn and Venus.
However most astronomers today classify a Christmas star as when Jupiter and Saturn align so closely during the last half of December so that they create a magnificent ray of light. It's pretty rare that these 2 line up at all from the Earth's perspective. It happens every 19.6 years. But the last time they were this close to create an incredible ray of light would have been March 4, 1226! As for when it would have happened around Christmas? I don't see any records for that.
Jupiter and Saturn will replicate that light show from 1226 this year on Monday December 21st! It will be at it's absolute brightest about 45 minutes after sunset. Here in Columbia SC that's about or 6:05 PM.
If you miss it, the wait will be significantly shorter for the next occurrence on March 15, 2080.