Originally Posted by DELTAFO

Not always.

You could be met at the jetway by a chief pilot telling you that you have been IA into a trip starting tomorrow that was supposed to be a day off. Or, you could be rerouted by crew tracking via ACARS to fly into a day off. Or, assigned a trip on reserve that flies on your off days during the transition from a month of reserve to a month of line (I know that technically isn't an IA but it's still working on days you thought you had off).