May the peace, mercy and blessing of Allah be upon you.

I would like to ask for the ruling on how correct it is to fast the Day of Doubt (if there is disagreement concerning the first day of Ramadan) for someone accustomed to fasting continuously, but who avoids the days on which it is prohibited to fast (Eid ul-Fitr, Eid ul-Adha, and the Days of Tashriq)?

Would the verdict be different for someone who fasts Mondays and Thursdays if one of them coincides with the Day of Doubt?

How correct would it be to fast only on Friday or Saturday for someone who is accustomed to fasting every other day, according to the practice of the Prophet Dawud (peace be upon him)?

May the peace, mercy and blessing of Allah be upon you also.

All praise is due to Allah, and may peace and blessing be upon the Messenger of Allah, his family, companions and true followers. To proceed:

Continuous fasting, if the intention behind it is to fast for all time, is disliked (makrooh). This is due to the saying of the Prophet (may Allah bless him and grant him peace), "May he who observes a perpetual fast never fast." [Agreed upon] It is also based on the hadith, "The most beloved [way of] fasting to Allah is the fasting of David (Dawud)," [Agreed upon] which was to fast one day and not fast the next.

As for fasting on the Day of Doubt, if it coincides with your habit of fasting, if you are fasting continuously for a certain period of time, like Mondays and Thursdays, then that would be permissible, due to the saying of the Prophet (may Allah bless him and grant him peace), "Do not fast a day or two before Ramadan, except for a man who has a particular habit of fasting; he should fast." [Agreed upon]

As for fasting only on Friday or Saturday, this is disliked (makrooh) according to many scholars, due to the saying of the Prophet (may Allah bless him and grant him peace) about Saturday, "Do not fast on a Saturday, except for that which has been made obligatory on you; so, if one of you find nothing but a grape-skin or a twig off a tree, he should chew on it." [Transmitted by Imam Ahmad, at-Tirmidhi, Ibn Majah, and al-Albani (may Allah have mercy on them)] The Prophet (may Allah bless him and grant him peace) said concerning Friday, "None of you should fast on a Friday unless he fasts [a day] before it or after it." [Transmitted by Imam Muslim (may Allah have mercy on him)]  

And Allah Almighty is more Exalted and more Knowledgeable.