Thus, every 20 years, Jupiter catches up to Saturn as viewed from Earth.
From the years 2000 to 2100 inclusive, as viewed from our planet Earth, these Jupiter/Saturn conjunctions (in ecliptic longitude) happen on these dates:
May 28, 2000
December 21, 2020
October 31, 2040
April 7, 2060
March 15, 2080
September 18, 2100
These great Jupiter/Saturn conjunctions recur in periods of 20 years. Each year, Saturn completes about 12 degrees of its orbit around the sun, whereas Jupiter completes about 30 degrees. Therefore, in one year, Jupiter closes the gap between itself and Saturn by about 18 degrees (30 – 12 = 18 degrees). In a period of 20 years, then, Jupiter gains 360 degrees on Saturn (18 x 20 = 360 degrees), therefore lapping the ringed planet once every 20 years.
So start watching Jupiter and Saturn now! And mark your calendar for the great conjunction of Jupiter and Saturn on December 21, 2020.
Bottom line: The upcoming great conjunction of Jupiter and Saturn will be December 21, 2020. This is a once-in-20-years event. In September, these two planets are in the sky when darkness falls. They are bright and easy to see. Charts and more info here.