Can I change which day I get the Activity.app weekly summary?
Looks like it probably isn't possible. In Apple Watch.app -> My Watch -> Activity there is a toggle that says:


  When this is on, every Monday you'll get a brief recap of your Move
  performance from the previous week, and you can adjust your Move goal
  for the upcoming week


Which to me suggests there isn't a setting for changing the day you receive the recap.