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Ditto what was said above, the Federal Reserve is closed on weekends and certain holidays. This means the necessary clearing process for deposits and debits between banks does not happen. Aside from the courts very few federal agencies, departments or whatever are open on weekends or holidays. Historically neither were many businesses either.
This being said some banks are 24/7. TD Bank for instance will accept deposits up to closing time (7PM NYC on weeknights) and credit accounts accordingly.
Banking activity is always one day behind anyway, so you really aren't missing that much.
They also get to play the float and gain a little bit of interest on your money prior to crediting to your account. It may be pennies (or dimes) but all those transactions add up.
It's a lot better than it used to be-5 business days for a government issued check, 10 business days for a personal/business check.