Background. Many contract provisions remain silent on the precise period of time within which something has to happen. The alternative for immediate or prompt action, acting upon the occurrence of an event or acting within 30 days after a notice was received, is to allow that an action must be undertaken without undue delay.
Why? There may be many reasons for using without undue delay:
the event triggering an obligation (or a right) to act is itself vague;
it is not foreseeable how much time will be needed in order to take the required (or allowed) action effectively and efficiently, though adequate preparation will be desirable in any case;
when a triggering event occurs, there will probably be no urgency to act immediately, in which case thoughtless action, merely to prevent contractual rights from lapsing, should be discouraged (but the opposite may also be true);
not providing any limitation in time creates too much uncertainty.