Planetary Great Conjunctions are rare astronomical events in which two or more planets align in the night sky. These events occur when two or more planets are in the same degree of the zodiac, and they can be seen with the naked eye. During a Planetary Great Conjunction, the
From Wikipedia
A great conjunction is a conjunction of the planets Jupiter and Saturn, when the two planets appear closest together in the sky. Great conjunctions occur approximately every 20 years when Jupiter "overtakes" Saturn in its orbit. They are named "great" for being by far the rarest of the conjunctions between naked-eye planets.