It depends on the show and when people have their vacations. There's no definitive answer. Since it seems like most shows that weren't dark did a matinee only this past year, I'd imagine unless a vacation is planned or they get sick or injured it'll be the regular people.
Many shows aren't performing July 4th this year, though some are doing matinees instead.
I'm sure the necessary number of people will be performing the show, though I'm sure most of us can't say that far ahead who will be on holiday.
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