I don't know. In the original script I would guess Sullust is the gathering point before the jump to Endor, but of course the EU goes totally over the top and has this to say:

http://starwars.wikia.com/wiki/Battle_of_Sullust_%28Galactic_Civil_War%29

"Reports of a Rebel fleet massing near Sullust" were first mentioned by Darth Vader in Star Wars Episode VI: Return of the Jedi, and the gathering was featured in X-Wing Alliance. The Expanded Universe never elaborated any further on the said event until Star Wars Battlefront: Renegade Squadron was released in 2007, giving the details of the battle.^[4] Jokingly commenting on the battle,^[4] game programmer Mark said:

"Don't listen to the old man when he says 'it is of no concern,' dude -- because the Renegade Squadron is plenty cause for concern."
?Mark (game programmer)