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Why is the stock market closed on the weekend?
While a lot of trading is executed by computers, a substantial amount is still done at the behest of humans.  Brokers managing accounts, Portfolio Managers, and Managers of Mutual Funds doing stock picks etc.  Those folks are still initiating a very large number of the trades (or at least one side of a trade).  And those humans don't work 7 days a week. it's not just computers talking to computers at the behest of other computers.  And even a lot of places that use computers to create models and such, there are still humans in the loop to ensure that the computers are not ordering something stupid to be done. I personally worked for a firm that managed nearly $20Billion in stock portfolios.  The portfolios were designed to track indexes, or a mix of indexes and actively managed funds, but with the addition of managing for tax efficiency.  A lot of complex math and complicated 'solver' programs that figured out each day what if anything to trade in each portfolio.  Despite all those computers, humans still reviewed all the trades to be sure they made sense. And those humans only worked 5 days a week.