I tried to schedule pickup with USPS.com for today (it's 1:20 am on Saturday). It says you can schedule any time before 2:00 am CST. I am in the Central time zone so it should work.
But when I get to the page to schedule pick up date and enter August 16 (today) it tells me I have selected "an invalid date". Out of curiousity I checked for pick up on the 18th and it worked.
So, why is it doing this? I will have to ask the postal clerk. I'm not counting on them having any clue either.