Instead of starting with a launch day and hoping that people will end up buying from you, why not build a community around your product, your niche or your industry before you start building your product? Before you even start deciding about what product to build?
Because it's hard.
Because building a community requires a whole different set of skills than building a product does.
But building a community before you build your product is probably the only way to succeed if you don't have hundreds of thousands of dollars to finance that big launch day...