You might be luck. The policy is that you can fly on any flight within 24 hours of your original flight (which the 8:05pm the night before is) and you can confirm that flight within 12 hours of ITS departure time, meaning starting at 8:05am on Sunday morning. If you are going for a flight 23 hours prior to yours you can actually call 12 hours prior to that, or T-35H from your original flight, as I understand it.

The main exception to this is that the weekend eSaver tix specifically exclude standby, or so I've been led to believe. I haven't read the full fare rules on those in some time now. But based on your Saturday/Monday schedule I'm guessing that's what you're going for, so maybe not so much luck for you.