Most small businesses open by simply becoming open. No date, no announcement, no reason for anybody to pay attention on that particular day rather than some other one.
Pick a date. Tell people three weeks out that it is coming. Tell them again the week of. Give the first week a reason to exist — a launch price, a limited run, something that expires.
You get exactly one first day. It is the only day where being new is interesting, and it is gone whether you used it or not.