What is incumbent upon me if I vowed to fulfill a pledge within a month, but I did not do so and the period expired?

Delaying a temporary vow beyond its specified time without a valid excuse is not permissible, and the one who does so incurs sin. They must repent and hasten to fulfill their vow. Some scholars, in addition, have mandated an expiation for breaking an oath (kaffarat yamin). Therefore, one must repent to Allah and promptly fulfill the vow, and it is more cautious to offer an expiation for breaking an oath to avoid scholarly disagreement.