The Dow Jones Industrial Average is an index that shows the average price of 30 large, publicly traded U.S. companies across many sectors. The index represents the performance of its constituents and is often used by investors and the media to portray an overall summary of the entire U.S. stock market. As it is a statistic and not an exchange or marketplace, there is no certain time the Dow Jones is open. However, the value of the Dow Jones fluctuates during each trading session as the prices of the underlying stocks fluctuate. Therefore, you could relate the trading hours of the index to that of the Dow Jones component stocks.
For example, if you invest in the SPDR Dow Jones Industrial Average ETF (DIA), the value of those shares fluctuates throughout the trading session as the value of the Dow Jones changes. You can trade your shares during those market hours just as if you owned shares of all the stocks within the index. You cannot buy shares of the actual index, though, because it is simply a statistic, not a security.
The Dow Jones components are all listed on the New York Stock Exchange (NYSE) or the Nasdaq, so the index follows the trading hours for these two exchanges. Both exchanges follow the same schedule of hours from 9:30 a.m. to 4 p.m. EST, Monday through Friday. The NYSE and Nasdaq both observe the same holidays. A current calendar is available on the website for either of the exchanges.