NASDAQ Trader reports that the NASDAQ officially opens for trading at 9:30 a.m. Eastern Standard Time. NASDAQ begins preparing to open at 4 a.m. when a computer starts entering the trades and orders that were received after market close the day before, according to HowStuffWorks.
HowStuffWorks notes that the computer also gathers imbalance information that has the potential to affect stock prices, such as if a company announced bad news after the market closed the previous day. At that point, an opening price is assigned to each stock.
Price information is released at 9:25 a.m. Eastern to dealers along with imbalance information at 9:28 a.m. Eastern before the market officially opens at 9:30 a.m. Eastern, notes NASDAQ Trader.