Nothing dramatic. A missed day is one square out of ninety.
Don't double up
Don't make it up by doubling tomorrow. That's how one missed day turns into three.
Plan90 moves what's left of the week onto the two lighter days at the end of it, so the work still has a home.
Never miss twice
The rule people stick with is simple: never miss twice. Once is life. Twice is the start of a habit, and not the one you wanted.