Thanksgiving
Thanksgiving would be the perfect day for civilization to collapse. There would be a sudden absence of basic goods and services, but people would be predisposed to be charitable to each other. Plus there would be leftovers.
Before that happens, I would like to declare a new holiday. It would be called "National Be Your Own Worst Enemy Day." Unlike other holidays, which don't normally prevent us from having hordes of enemies who want to destroy us, this would be a national day of recognition of the fact that it's mostly not them that are out to get us. It's us.
So, you know, when civilization collapses, we'll know who did it.