It begins sometime after the end of 2.0's last main scenario quest. You need to finish what was all of the main scenario (Praetorium), achieve the highest rank in your Grand Company, and dispatch the hard mode versions of Ifrit, Garuda, and Titan to open up the Binding Coil of Bahamut. All the turns and subsequent coils open sequentially with dialogue driven quests between the Binding Coil of Bahamut and the Second Coil of Bahamut, and between the second and the Final Coil of Bahamut. At this point the first two coil areas (turns 1-9) can be entered through the duty finder and farmed all day long. The Final Coil of Bahamut allows you to complete each turn once weekly at most.