The Dow Jones Industrial Average (DJIA) 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 as an overall summary of the entire U.S. stock market.
As it is a statistic and not an exchange or marketplace, there is no one 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 in it fluctuate. Therefore, you could relate the trading hours of the index to that of the Dow Jones component stocks.
Of course, you cannot buy shares of the actual index, because it is simply a statistic, not a security. But you can buy shares of mutual funds and ETFs whose portfolios mirror the DJIA. For example, if you want to invest in the SPDR Dow Jones Industrial Average ETF (DIA), you can trade your shares during market hours just as if you owned shares of all the stocks within the index. The value of the ETF fluctuates throughout the trading session as the value of the Dow Jones changes.
The Dow Jones components are all listed on the New York Stock Exchange (NYSE) or 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, and observe the same holidays.