The day before. If you switch to a different resort, be proactive and once you make the change, call II and the resort after a couple hours to ensure II has sent over the new reservation to the resort.
There was a recent post on FB about a woman that booked a Hawaii resort on a Saturday night for a Sunday arrival. She got there an no reservation, she had to pay cash for the night. On Monday once II opened up again, they straightened it out. There explanation was since they were not open on Sunday and the exchange happened late Saturday they didn't change the names over.