Great conjunction
The conjunction of Jupiter and Saturn, roughly every 20 years.
It is the oldest and most used long cycle in mundane astrology; ninth-century Arabic astrologers used it to mark out dynasties and eras.
Successive great conjunctions stay in signs of one element for two centuries or more before moving on. The one in December 2020 fell at the very start of Aquarius, hence all the talk of an "air era".