Each guild has one time slot per week.
https://wiki.project1999.com/Plane_of_Sky_Raid_Rotation
Also, note that Noble Dojorn is not always up. His loot is quite rare, if you’re looking at the horn.
Also, pretend Overseer of Air and Hand of Veeshan don’t exist. All guilds agree to leave the OOA up, because that causes more Noble Dojorns to spawn; and Hand of Veeshan only spawns when OOA is killed (so, never).
So if you’re aiming for the horn, note that it will probably take a lot of DKP with whatever Guild you do Sky with.
If Sky seems weird, that’s because it is weird. It’s full of quirky, convoluted, and inconvenient mechanics.