It's too bad your shift problem didn't happen until the big day.
A local rider was dropping the chain when he rolled the bike backward while standing next to it. (This makes the crank turn backward). It was happening once or twice a ride, very annoying.
Now it's fine. All these were fixed at the same shop visit, so one or more was the cause:
1. a slightly bend derailleur hanger
2. a worn chain, somewhat overdue for replacement
3. front derailleur adjustment