All days off are moveable except for 3 (sometimes 4) 'golden' days off per month. There are some rules restricting how many and how often scheduling can move days. In practice, for me it's uncommon for Scheduling to move days off. I think they've done it to me twice in the past year. A pilot can in theory move a moveable with Scheduling's concurrence, but that is rarely done (I've never done it). That said, the Chief Pilots have been extremely flexible when I've needed a day off for things beyond my control and have given me days off out of my vacation bank every time I've needed them. Premium pay, again, is at the sole discretion of Scheduling, and it's only given when they are short on manning, and *only* if you would not have been on the hook to fly the sequence anyway (eg, Sched may call on your days off and beg you to fly).