,احسن الله إليك If someone is regularly doing a voluntary fast like the fast of Mondays and Thursdays or the Fast of Dawud عليه السلام, and they happen to break it for some reason, is it permissible for them to make it up on a different day?

Imam Ibn Baz said: He is not required to make it up, but it is recommended for him to do so. If someone intended to fast on Monday or Thursday and then broke the fast without a valid excuse, there is no blame upon him. However, making it up would be better.