Is it prescribed for someone who is accustomed to fasting Mondays and Thursdays, or something similar, to make up a day that he missed due to an excuse or without an excuse?

One who is accustomed to observing a supererogatory fast, such as on Mondays and Thursdays, and misses it, it is recommended for him to make it up. This is the well-known position in the Maliki and Hanbali schools. They cited as evidence various hadiths, including the Prophet's (peace be upon him) i'tikaf (spiritual retreat) in Shawwal as a substitute for Ramadan, and making up the two rak'ahs of Fajr after sunrise. Imam al-Nawawi and Ibn al-Qayyim mentioned that this indicates the permissibility of making up regular supererogatory prayers (sunan rawatib). Ibn Baz and Ibn Uthaymeen both chose the opinion that it is permissible to make up supererogatory acts of worship if they are missed due to an excuse.