I know that there are Compliance Guru's out there (you know who you are, and we love you), that can answer whether or not there is a Reg Requirement, but from a customer service standpoint I would think you would want customers to know not to expect to cash large checks on a weekend - the notice might take some of the heat off of your teller staff having to tell customers.
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My comments are absolutely no reflection of, nor influenced by, my employer - take them at your own risk.