A "lay-down date" is also called an "on-sale" date, but please don't mix it up with "release date" or "publication date".
The last two are soft terms used by publishers and booksellers to indicate an approximation of when books will be on shelves. The first two are "put it on sale before this date and there will be legal repercussions" terms.
Most books do not have lay-down dates.
1. The only books that get a firm on-sale date are books for which the publisher is expecting (or just really hoping for) a strong and immediate demand.
2. The vagaries of shipping mean that some bookstores will always get their shipments ahead of other bookstores.
3. Which means that if bookstores who get the next Harry Potter (eg) early are allowed to start selling them immediately, the other bookstores are S.O.L. and will lose a bunch of revenue to the bookstores who were lucky enough to get their shipments first.
4. In which case the publisher will be immediately inundated by furious calls from the bookstores that haven't gotten their books. Furious. Inundated.
5. Thus: lay-down dates. Everyone gets a chance to receive the books, and then everyone has to wait for the starter's pistol to start selling them.
6. Plus! The more the publisher can pack the first wave of sales into a single week, the better their chance of getting on bestseller lists. Do publishers want to be on those lists? Hell yes, they do.
And now for a question I can't answer: Why are most lay-down dates on Tuesdays?
Nevermind the (sometimes absurd) theories that Millions found through the intensive method of (ahem) googling it. Go read it from someone who actually did their research.