Not so grand
Grand openings are severely overrated. So are product launches and galas of all sorts.
Make a list of successful products in your industry. Most of them didn't start big. Not the Honda Accord or Facebook, not Aetna Insurance, not JetBlue or that church down the street. Most overnight successes take a decade (okay, four years online).
The grand opening is a symptom of the real problem... the limited attention span of marketers. Marketers get focused (briefly) on the grand opening and then move on to the next thing (quickly). Grand opening syndrome forces marketers to spend their time and money at exactly the wrong time, and worse, it leads to a lack of patience that damages the prospects of the product and service being launched.
Non-profits do the same thing when they spend months planning an elaborate gala that takes all the time and enriches the hotel and the caterer. Far better to spend the time and money building actual relationships than going for the big 'grand' hit.
The best time to promote something is after it has raving fans, after you've discovered that it works, after it has a groundswell of support. And more important, the best way to promote something is consistently and persistently and for a long time. Save the bunting for Flag Day.