Answer:
(c) Special rule for banks
Any bank (as defined in section 408(n)) shall surrender (subject to an attachment or execution under judicial process) any deposits (including interest thereon) in such bank only after 21 days after service of levy.
If the 21st day is Saturday and you are not open or Sunday - you would surrender on Monday. It does not say surrender on the 22nd day - just after 21 days.