Forget about what your predecessor wrote down and read the instructions to IRS Form 668A(c):
Money in banks, credit unions, savings and loans, and similar institutions described in section 408(n) of the Internal Revenue Code must be held for 21 calendar days from the day you receive this levy before you send us the money. (Emphasis in the original)
Regulation CC excludes federal holidays because they are not "business" days. There is no regulation that suggests they are not calendar days.
P.S. Examiners do not review compliance with IRS levies. They know absolutely nothing about the subject.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.