Mont. Code Ann. § 32-2-942

No law of this state may affect the validity of, or render void or voidable, the payment, certification, or acceptance of a check or other negotiable instrument or any other transaction by a mutual association in this state because it was done or performed during any time other than regular banking hours or on a legal holiday. A mutual association in this state, which by law or custom is entitled to close at 12 noon on any Saturday or for the whole or part of any legal holiday, may not be compelled to remain open for transaction of business or to perform any of the acts or transactions in this section on any Saturday after 12 noon or any legal holiday except at its option.