You might know that overnight permits are picked up the day before, at 10:00 am. By 11:00 am the reserved permits not picked up are also available
Overnight permits can be picked up as early as 48 hours in advance. If you do not pick it up by 10 a.m. the day of entry, you lose the permit. This was confirmed by Walter at the Eastern Sierra InterAgency Center this morning. This is the rule for MWMT, but I am not sure if it is the same for other trail heads as well.