4am-9:30am and 4pm-8pm. Access to extended hours is determined by your brokerage. Institutional investors have full access to extended hours.
What's Wrong with the Normal Hours?
"Companies are strategic about their announcements of important information. They don't like to make announcements during the regular trading session because it could cause a large knee-jerk reaction that misrepresents the true value of their stock. If a company was to announce its last-quarter earnings and they were worse than expected, a large-scale move out of the stock could result in big losses that may not be warranted.
Although announcing this information before or after the close of the trading day may still cause a large reaction, by the time the market opens, investors and analysts will have read the entirety of the announcement, causing the stock price to better reflect fair value."
( http://www.investopedia.com/financial-edge/1112/trading-in-the-pre--and-post-market-sessions.aspx#ixzz3fUmDjAms )
