In the Star Wars universe, dates are usually given as "[number of years] BBY" or "[number of years] ABY" - that is, X years "Before the Battle of Yavin" or X years "After the Battle of Yavin". The Battle of Yavin, of course, marked the destruction of the first Death Star.
This is puzzling, because the Empire was still going strong after Yavin, whereas the Battle of Endor was catastrophic for Imperial forces - the second Death Star was destroyed, the Emperor and Vader were killed, the Imperial Fleet was scattered, Imperial leadership fell into disarray and in-fighting, the Rebellion emerged as the dominant power in the galaxy, etc.1
Out-of-Universe, "BBY/ABY" makes sense, because the climactic moment of the first Star Wars movie is the Battle of Yavin, and is familiar to everyone who has seen the film.
In-universe, it makes less sense, because the consequences of the Battle of Yavin were far less pronounced than the consequences of the Battle of Endor.
So why does the galaxy far, far away use "BBY/ABY" rather than "BBE/ABE"?
1 The Battle of Jakku was arguably even more significant, since it marked the end of the Empire as a sizable military entity, and ended with the surrender of most Imperial forces and resources. However, the Battle of Jakku only entered canon recently, long after "BBY/ABY" was established, so it is probably too late to switch to "BBJ/ABJ".