I’ve heard this for years and can’t find any citation to either prove or disprove it…“It is illegal for a bank to be closed more than three days in a row.” I suspect it is an urban legend and has more to do with tradition than law but I keep running into blind alleys trying to research it. I’m now turning to the ultimate source to get to the bottom of this burning question.
Banks are regulated by various authorities, usually a state. A quick look at New York State Banking Law shows nothing said about hours of operation or days of operation.
I’ve never heard the assertion before.
Nothing that I can find in the Canadian Bank Act covers hours/days of operation, but see http://www.fcul.org/cr_nov00.html which mentions “not closed for more than three days” as a federal legal requirement for for US banks.
Sorry this isn’t very helpful, but I’m sure some banking regulators do have requirements for trading/banking computer system uptime, which is somewhat related to the question. I can’t find anything in a quick Google and search of the (UK) FSA, having said that.
I’ve just spent the better part of an hour trying to find the answer where I thought it would be, but I’ve about given up 
In my former life I was pretty involved with auditing and accounting within the banking system, and I recall the same “3 day rule”.
I’ve looked through the online resources for the Federal Reseve Bank and the Office of the Comptroller of the Currency, and can’t come up with specifics.
However, as I try to file through the cobwebs of my mind, I believe that a Federally chartered bank in the U.S. has to be open any day that the Federal Reserve Bank is open, and it might just be that the FRB can’t be closed 3 days in a row (like the day after Thanksgiving, a lot of the U.S. is “closed”, but banks are always open), so Federal banks can’t be closed either. Naturally, I can’t find a cite for that, either
Also, banks chartered under a State charter instead of a Federal charter are likely open according to State instead of Federal law.
Sorry this hasn’t provided any concrete evidence, but perhaps there are some banking regulators here who can “show us the way.”