If it's a listed stock (NYSE/AMEX), the specialist will determine the opening price. He will arrive at his figure by looking at the open book between 8am-930am, before the market opens, and also by getting a feel for the institutional buyers and sellers who are represented on the floor by the floor brokers. I believe he then opens the stock where the most open orders would get filled.
If it's a Nasdaq stock, investors, traders and marketmakers will post their bids and asks via ECN's just as soon as the ECN's turn on for the day. I don't think they are 24-hours yet, someone please correct me if I am wrong. I think they turn on around 7:30 or 8:00am. At first, you are most likely going to see a wide spread at this time. Maybe like $55.00 x $70.00 A few brave souls will begin first thing in the morning by bidding $55.00, feeling relatively confident this is a safe bid. The first few sellers of the morning are also equally confident sitting up at $70.
Then as the seconds and minutes tick by, the spread gets narrowed by more bidders and sellers until trades finally start taking place.
Chances are, if a company announces blowout earnings in the evening, the stock is already trading on the ECN's up until 7pm or 8pm, or something like that. I'm not sure of the exact times when the ECN's are open.
So that's my understanding of it anyways. Someone please correct me if I am wrong, or if I missed anything.
Enjoy, and good trading to you.
+-*/ Math_Wiz