Jupiter and Saturn are set to align to create the first visible 'Christmas Star' in 800 years.
The celestial event, known as the Great Conjunction, appears every 20 years, but this year's will be the closest the two planets have been since 1623.
However, it will be the first conjunction to be visible to the naked eye since 1226, with the next expected in 2080, so you won't want to miss it.
According to Dr Stuart Clark, the 'star' should be visible looking south-west at around 5pm (GMT) on Monday (21 December), and will be joined by a bright crescent moon.
Patrick Hartigan, astronomer at Rice University, said it was a special event.
He said: "Alignments between these two planets are rather rare, occurring once every 20 years or so, but this conjunction is exceptionally rare because of how close the planets will appear to be to one another.
"You'd have to go all the way back to just before dawn on March 4, 1226, to see a closer alignment between these objects visible in the night sky.
"On the evening of closest approach on Dec 21 they will look like a double planet, separated by only 1/5th the diameter of the full moon.
"For most telescope viewers, each planet and several of their largest moons will be visible in the same field of view that evening.
"The further north a viewer is, the less time they'll have to catch a glimpse of the conjunction before the planets sink below the horizon."
It is called the 'Christmas Star' or 'Star of Bethlehem' as some believe that the star that guided the 'three wise men' to Jesus could have been an extremely rare triple conjunction of Jupiter, Saturn and Venus.
When the planets align, they create a bright light, which gives them the appearance of a huge star.
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