Roster pattern is rock solid outside your twice-yearly 'reserve period'. This is the new 3 week standby period that goes with the equally new 5/2,5/4 roster pattern. Everyone knows when their reserve period falls.
Rosters are published by the 17th of each month and emailed to your company email account (accessible from home). Rosters have been produced on time every month for almost two years - or at least that's the general claim. It's true in my case, anyway. Stability is determined by your base. My base enjoys almost 100% stability, others do not.
You could still see a standby out side your reserve periods. We also have 'contactables' which could be changed to a duty in advance. But once you've checked out the day before they essentially become days off since the company has to give at least 10 hours notice prior to a duty. This is not always the case though, but nearly always. 'Contactables' are given when there are surplus pilots (for whatever reason) and rostering wish to safeguard your duty hours.
It's not as bad it often sounds here on this website, though alot depends on your base.
FQ