This particular event hasn't been seen on Earth since 1623.
Right now (provided it is night time when you're reading this), even to the naked eye, Jupiter and Saturn are two visibly bright dots in the sky, getting incrementally closer to each other every night.
However, before the end of the month, the two "stars" will get so close together that they're expecting to create a combined, much-brighter light, which is being referred to as "the Christmas Star".
The celestial event will be visible from pretty much everywhere on the planet (provided there isn't cloud cover or heavy light pollution), but we wanted to know where, when, and how we could get the best glimpse at this once-in-a-lifetime event from Ireland.
We spoke to David Moore, the editor of Astronomy Ireland Magazine, and he told us where to look to get the best view:
"It will be happening fairly low down on the horizon, so you'll need to find yourself a place with no built up areas around you. It'll be on the horizon, looking towards the southwest, so be looking in the direction that the sun sets in."
He also told us when is the best time to check it out:
"The best day will be the 21st of December, which just so happens to the shortest day of the year, the Winter Solstice, but that is purely a coincidence, it isn't linked in any way. And the best time to see it will be very soon after sunset, during a strong twilight.
"The sun is setting at this time of year around 4pm, so you'll see Jupiter pretty much straight away, and then you'll be able to see Saturn within half an hour or so, but from Ireland, between 5pm and 6pm would be the best time to see it."
And, finally, he told us what to expect to see:
"Right now, they're already close enough to be quite spectacular. On the day, they'll be about one-fifth the width of a full moon, and they'll be close enough together that it will be about the limit of the human eyes to be able to separate the two objects.
"You can buy a decent pair on binoculars now for about 20 euro, and they will be about 100 times better than your eyes and will help you to separate the two objects in the sky, and you'll even be able to see the moons of Jupiter.
"If you use a telescope, you'll be able to see Saturn, Jupiter, and the moons of Jupiter altogether, which is something that no-one has ever seen; this may never have happened before, that we could see these together so clearly. The last time this happened was in 1623, and Galileo's telescope was only about as strong as our present day binoculars."
If you miss it this time, then don't worry, it won't be as long a wait until it happens again.... in the year 2080.