Jupiter and Saturn have gradually moved closer to each other over for months, and on December 21, the two worlds will be at their closest, around 1/5 of a full Moon apart. This is close enough that many telescopes may be able to see both planets at once in the same field of view! These conjunctions occur every 20 years, and this is an especially close one!
Keep in mind that while the two gas giants may appear close, in reality they are hundreds of millions of miles apart. This will still be quite a striking sight, but you will need to look fast as both planets will set shortly after sunset. Look above the western horizon after sunset for these bright, close planets- a clear view will help!