New comic book day on holiday weeks?

I understand why New Comic Book Day (usually Wednesdays) gets pushed to Thursday on a week when there's a Monday holiday (such as Memorial Day).

I accidentally walked into my local comic shop yesterday, and the owner told me "Come back tomorrow" - but it seemed like the new books were already up on the shelf - but he couldn't sell them yet?

Is there some rule that prevents him from selling the books a day early (which would be on the regular day) during a week with a holiday?