Does each volunteer have a job?

A successful event happens when all the actors know what their role is.

Have a list of tasks and jobs ready, which you can then distribute among the volunteers on the day. Update the list as tasks are identified throughout the day and make sure there is only one designated person that is in charge of the list.

If each volunteer know exactly what their job is, there is less standing around, less overlap between tasks and less time wasted. And more time for the volunteers means they can enjoy more of the event.