I assume it would be the opening auction. At the start of trading every day (and on every interday restart and close of day) there is an auction held. All inter

I assume it would be the opening auction. At the start of trading every day (and on every interday restart and close of day) there is an auction held. All interested participants enter market or limit on open orders from 5 to 15 minutes before the auction (depending on which auction). The exchange will periodically publish what are called imbalance numbers that show relative buy/sell pressure and what price the auction would go off at if it happened that instance. When the auction does go off, the price is chosen to maximize volume. This is how we get opening and closing prices of a stock. It isn't just the last price traded. In this context, the those that have stock (including the company itself) would probably have their shares moved to a brokerage and then they can sell directly to the public who wish to buy. Basically the same that would happen with the first day of public trading on the secondary market after the initial offering.