The market is mostly closed on Monday's to begin with with the exception of a few shops.(DiBruno's is one of them)I would have to venture a guess that it will be closed for the most part if not completely on Labor Day. It should be open on Sunday until about 2pm. It is definitely somewhere to visit when in Philly, good luck.
PS. You may want to call DIBruno's and ask if they will be open, if they are not, chances are that no one will be.