There are some old religious "Blue Laws" still on some state law books that prohibit car sales on Sundays...because they apparently looked at that as a vice like buying alcohol hehe.
Not aware of anything for Saturdays. There may be some NY oddball law out there. May have something to do with the titling/registration. Or something as simple as if they're getting the car Friday, they just want to reserve Saturday to do their prep on it before delivering monday.
Edit: or not since the above poster is also a New Yorker.

So probably just the wanting time to prep bit. Or the sales manager wants to take saturday off or some such heh. If you've already signed papers, they don't have much incentive to bend over backwards to get your sale.

Shouldn't do that until the car you want is there ready for pickup.