Lesson learned: I searched for a reservation by putting in my vacation dates first, so I didn't check this host's calendar when it said the dates were available. I used the Instant Book feature and didn't realize until after it went through that the weekend rate is $30/day more expensive. I immediately called Airbnb to cancel the reservation. I also contacted this host to let him know the error and asked if I could cancel the booking with him. He said it was not problem. I was refunded by Airbnb for the second day of the booking, but this host kept the first night's payment plus cleaning deposit.
Moral of the story: check the host's calendar (even if the dates are available) to see if they charge more for the particular dates you want, because it won't say so until you are charged, and then, obviously, it's too late.