Edited Message (for sense)
My guess would be that he went on because he'd just had his "put-in" rehearsal and the show wanted him in that night. Friday afternoon (though sometimes Thursday) is the usual day for put-in rehearsals. The cast member going in to the show uses full costume, show props, wig/hair make up, etc. Most of the put-ins I've witnessed have full lights and, sometimes, full orchestra.
In most shows, I've seen the actor go on that same night after the put-in. Sometimes there's a single day time rehearsal after the put-in for corrections and the first performance is then the day after the put-in. Having said that, I've also seen a few shows where the actor being put-in was scheduled to start performing on the following Tuesday.
Updated On: 5/24/17 at 06:22 PM