We TRY to explain this in the Wilderness Permit Options (link to the left). From
Step 6:
For an overnight permit, show up at the Visitor Center by 11 am for no-shows and cancellations. No-shows for entering on the current day become available at 11 am. Cancellations that have not been re-reserved for entering the FOLLOWING day also become available at 11 am.
The above is for overnight (backpacking) permits. However, I was there several weeks ago picking up my overnight permit at 8 AM for the same (current) day, and tried to add another person. I was told none was available. But my friend walked in fifteen minutes later, and was given a permit because some group had come in and picked up some of their reserved permits and had released one or more. So walking in at 8:15, he was able to pick up a cancellation. Not sure if that will be the case later in the season.
So the 11 AM time for overnight (backpacking) permits is for the
same day, not next day. But it IS possible that there may be some available for the NEXT day available at 11 AM as well.