I already had a client before I even registered my business with the county. Just recently I had another client wanting my web hosting service. But I've told them to wait or find someone else because I haven't had gotten everything else prepared.
So, in my case, I'm guessing I'd say my first day of business is when I am ready to take in clients and offically open my virtual door to the public, ie website, etc.
Interesting thread by the way!