I was going from Clt to Phl last Sat. morning when the fog was in Clt. Our Capt. was commuting in from Knoxville and deverted to Avl. He should have come in the night before. The F/O made the PA about what was going on. US tried to get another Capt. and actually found one(pulled him off of his trip). We pushed back and were starting up, when the new Capt. came on the PA and said we were going back to the gate. Seems he was a International Capt. and there weren't any other Int. Capts. to take his trip later. So we went back to the gate while they tried to find another Capt. It was pretty embarrassing. After another 30 minutes, we finally got another Capt. I won't type the comments that were said around me. Hopefully, the original Capt. will get a good talking to about not being there the night before and not having a back-up flight.