How do I mark a day I'm not going out without breaking my recurring plan?

Tap a plan on that day in the Schedule tab to open it, tap More (•••), then Delete → Delete This Plan. Do this for each plan on that day. Only that day is removed, and a recurring plan keeps going from the next day forward. To go out at a different time that day instead, tap Edit, change it, and choose Only this date when you save.