Suppose I own a stock that closes at $10. I then put out a sell limit for 12. Good news overnight, and the next day, pre-market, it's trading around 20. When the market opens will I sell for the 20 or will someone get lucky and get my shares at 12? Thanks!
Depends what it opens at. You will get a minimum $12. If you're watching the stock an option would be to change your limit order to a limit-on-open order. That way you're guaranteed to get the opening auction price.
Also probably depends on who is processing your order. It is pretty conceivable you would get filled at 12, then the counter party sells at 20 nano-seconds later.
If your order is only active during regular trading hours you will get the open print or very close to it (depending on how your order is exactly handled by the broker). If your order went active any time during premarket, you will get the bid price at that moment if >12 or get executed at 12 if it ran through your limit after your order went active. Premarket starts at 4am, but trading is light till around 7am when it picks up a lot as tda and others join. Big news usually either happen right after market close or around 7am till market open. Usually not 10pm-4am.