Most plans meet a week where the moment you chose isn't there. Deciding beforehand what you'd do then makes that week part of the plan instead of a break in it.
Three ways to bend instead of break
A smaller version. Ten minutes instead of thirty; one page instead of a chapter.
Another moment. "If Tuesday is full, Thursday morning."
A pause. Sometimes the useful choice is to stop for a while. Pausing is a decision, not a failure, and it can be undone.
In UpWill
Write the second option into the next step ("If Tuesday is full, Thursday morning"), or note what tends to get in the way when you set the plan up. A plan's page lets you pause it and resume it later; a paused plan keeps everything you recorded. If a week was full, a note saying so is a record too.