The First Jedi Purge took place some time after the Jedi Civil War, from 3,955–3,954 BBY until 3,951 BBY. This purge saw nearly the entire Jedi Order disappear from the galaxy; the Order publicly disbanded and most of the survivors were killed by the Sith.
The conflict was interrupted by the emergence of Darth Symore, a fallen Jedi who was powerful enough to finish both the Jedi and the Sith. Symore eliminated the surviving Jedi Masters one-by-one before turning his attention to eliminating the Sith. After Symore left the galaxy, the Jedi rebuilt their numbers, managing to pull themselves from the brink of extinction.