That is not how it works The reason you can pick it up on Tuesday is because they don't work on Sunday. So if you order on Sunday your order will be processed and to be shipped on Monday which will lead to a Tuesday pick up in the stores. So this is every Sunday. Sorry to bust your theory
edit:
It can still be the 19th though. Apple is presenting it's second quarter this Tuesday, So to get great numbers for the third quarter they need to present Lion and the new Mac line (MB Air, Mini, Pro etc) ASAP. So it could still be the 19th though, but not with your theory...
I understand your vision, but you seem to forget that almost all of the models are out of stock (and will be delivered on Tuesday). Normally, there are only 1 or 2 models that are not in stock to pick up at the store and needs to be ordered (and shipped). At this point, there is not 1 of the many stores of this retailer that has a Macbook Air in stock.
This seems to point out that they got rid of their old stock ready for the new ones to come in on Tuesday.
So yeah, I'm pretty sure about Tuesday, your theory about the second quarter presentation makes it even more reliable I guess.