An extra bit of information that hasn’t appeared in other answers:
In addition to the transfers between banks being blocked in weekends by the federal Reserve, many smaller banks are unable to do internal operations in weekends. This is because they don’t actually own the software or computers that their bank runs on, but instead pay a company to do that for them. The largest company, FiServe, actually does turn their machines off for the weekend (or did in 2017 when I was working for one of their clients)
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