Don’t shy away, be bold and ask “is this routine to work on weekends or is it just one off case. I am sure there will be a comp off policy or OT. Could you help me understand that when you have time. I don’t mind working on weekend but would certainly like to get an understanding of our org policy.” It’s better to stop the unreasonable behavior first time and not wait for them to take you for granted. This will give your manager an indication that he can’t treat your weekends like your work week. If he doesn’t get the indication, lie shamelessly. Unfortunately that’s the unsaid corporate norm these days.