The Spacers' District existed on the planet Athus Klee. Before 10 BBY, during the Galactic Empire's rule, the Besalisk Dexter Jettster landed on Athus Klee utilizing a star-streamer. The starship's passengers rapidly exited and went into the Spacers' District; before Jettster could even exit the ramp of the ship, every single inn in the vicinity was completely occupied. Jettster would eventually write about this event in the book that became the Smuggler's Guide.
The 2018 reference book Star Wars: Smuggler's Guide, penned by Daniel Wallace, contained the initial mention of the Spacers' District.