If you want to start hiking on 8/31, you need to be there when they start giving out walk in permits for 8/31 at 11 am the day before. If you want to start on 9/1, at 11 am on 8/31.
If you want to be at the front of the line to ensure you get permits, you get there earlier in the morning and wait. Those are very popular trailheads. If you get there at 10 am, they start to give out unclaimed reserved permits for that same day. So you could start on 8/31 if you show up early and are lucky enough to be there on a day when someone fails to show up to get the permit they paid for.
https://www.nps.gov/yose/planyourvisit/wildpermits.htm