It's the first time the phenomenon will appear in the night sky in 800 years.
An extremely rare bright light, known as the "Christmas star" will be visible in the Irish night sky later today.
On the winter solstice, Jupiter and Saturn will be closer together in the sky than they have been in 400 years - creating the 'Great Conjunction'.
While it's been 800 years, since it's has happened at night, meaning tonight's event is an extremely rare occurrence.
From Earth the two planets will appear to be very close together, despite being hundreds of millions of miles apart.
The event is similar to the Star of Bethlehem, which is said to have led the three wise men to the nativity scene.
Stargazers are being told to look at the south west of the sky at around 5pm to catch a glimpse, and if you have binoculars all the better.
Space Commentator Leo Enright told Newstalk Breakfast about the event and how best to see it.
"Unlike any winter solstice in 800 years, Jupiter and Saturn - the two biggest planets in the solar system - are going to be 'kissing cousins'. They're going to be right beside each other in the sky this evening, just at sunset'', he explained.
"If you haven't great eyesight and you're looking with your regular eyes, you'll probably have trouble even seeing there are two planets in that bright spot in the sky in the south-west."
He encouraged people to look at the sky early in the evening if they want to spot the conjunction, as they will only be in Irish skies in the early evening.
He said: "If you go out at 4pm... if you had binoculars you'd probably be able to pick them out low in the south-western sky.
"At about 5pm, when the sun just about disappears, it should be visible to anyone who isn't clouded out.
"You really want to get into a position where you can see clearly down to the south-west, and look for this beautiful appearance of these two huge planets."
