I believe it is based on your check in date. You could separate your ressie--check in 19th & check out on 21st. Check back in on the 21st under new ressie for the discount for the remainder.
Or you could just switch your dates to arrive on the 21st.
Good idea (we had the same idea ) and, nope, couldn't get it. We're going August 19th-21st and it starts on the 21st, so we took a pass and are staying off-site instead.