Chapter 12 "Fear of Insurrection"

A muster is a gathering of military groups for a specific reason. This would include inspections, assemblies, and parades. We can infer that the slaves thought the muster was going to be a parade, as they equated the occassion with a holiday. 

It was always the custom to have a muster every year. On that occasion every white man shouldered his musket. The citizens and the so-called country gentlemen wore military uniforms. The poor whites took their places in the ranks in every-day dress, some without shoes, some without hats. This grand occasion had already passed; and when the slaves were told there was to be another muster, they were surprised and rejoiced. Poor creatures! They thought it was going to be a holiday.