Nothing in any law of this state shall in any manner affect the validity of, or render void or voidable any transaction by a bank in this state because done or performed during any time other than regular banking hours; Provided, that nothing herein shall be construed to compel any bank in this state which by law or custom is entitled to close at a fixed hour on any day or for the whole or any part of any holiday, to keep open for the transaction of business on any day after such hour, or on any holiday, except at its own option.