Marlboro_2002, I will give you the example from easyJet as it the schedule I work under. It is different in other companies.
Our roster (for pilots) is 5 early check ins, 3 days off, 5 late check ins, 4 days off. This pattern is repeated for 17 weeks followed by 4 weeks of random rostering. Then the fixed pattern starts again.
The June roster is published on the 17th of May, and the July roster on the 17th of June and so forth. However, you know your days off a year in advance (except for the 4 weeks of random rostering), but on the 17th of a month the next month's duties are revealed.
An example could be: Day 1: 4 sector day, day 2: 2 sector day, day 3: 4 sector day, day 4: stand by, day 5: 4 sector day. So, a stand by duty is on a working day. The company can NOT change your days off!
When you are off, you are completely off and do not have to think about the company at all.