How is it possible to combine fasting on alternate days (the fast of Prophet Ayyub, peace be upon him) with fasting on Mondays and Thursdays (the fast of Prophet Muhammad, peace be upon him)? And is it permissible to fast for an entire week except for one or two days?

The best fasting is the fast of the Prophet of Allah, David (peace be upon him), which is fasting one day and breaking fast the next. It is established that the Prophet (peace and blessings of Allah be upon him) consistently fasted on Mondays and Thursdays and encouraged it, because deeds are presented on these days, and he loved for his deeds to be presented while he was fasting. Scholars differed as to what is better: whether to consistently observe the fast of David, or to combine it with fasting on Mondays and Thursdays.

Perpetual fasting (fasting throughout the year) is not disliked for one who breaks fast on the forbidden days, does not neglect any right, and does not fear harm. However, the best is to fast one day and break fast the next. One must be cautious of what cannot be sustained, for the most beloved deeds to Allah are those that are continuous, even if they are few.