The plan should name what happens if its most important assumption is wrong.

Every plan rests on one thing being true, and usually nobody says which thing. The venue will be free, the money will come through, the person will say yes, the weather will hold: whichever it is, the rest of the arrangement is balanced on it, and the moment it fails everyone improvises badly under pressure. Naming the assumption out loud is uncomfortable because it admits the plan could fail. That admission is exactly what makes it survivable.

Write down the single condition your current plan depends on most. Decide now, calmly, what you will do if it does not hold.