You can try calling and seeing whether a human representative can help you knock off the first day of the reservation, but it’s possible you might just be stuck. Whether a cancelled reservation goes back into points inventory or not is pretty inconsistent from program to program. If you’re booking something you suspect might be a competitive reservation and you think there’s risk of changes, it’s generally advised to book individual days; as long as there’s no bulk pricing for a longer stay, there’s no downside beyond a little extra time booking.