Contingency
A rain plan you cannot walk is not a plan
Pretty is still not a plan. Weather, overflow, and curfew are votes. If the fallback is a sentence, the evening is a one-way bet.
What this is
A fallback has a place, a capacity, a route, and a person who calls it. 'We'll get a tent' is procurement, not design. 'Hold in the barn' is a plan only if you have walked it with the headcount.
What guests remember
Not your Pinterest. The hour they stood in rain, or the bathroom queue, or the DJ that ended because of a neighbour. Contingency is hospitality under pressure.
A reversible test
Name the most likely interruption. Walk the fallback once. If a second person cannot describe it without notes, it is still paper.