How does a Muslim fast the fast of David (peace be upon him), given the prohibition of singling out Friday for fasting, and was Friday not prohibited from being singled out during the time of David (peace be upon him)?

The fasting of David (peace be upon him), which is fasting one day and breaking fast the next, does not contradict the prohibition against singling out Friday for fasting, because he does not intend to fast Friday per se. Sheikh Ibn Uthaymeen clarified that there is no harm in fasting on Friday or Saturday if it coincides with days of fasting that are not intended to be singled out. As for the ruling on singling out Friday for fasting in the law of David, we do not know anything about the practical rulings in his law, for every prophet has a law and a methodology, and their beliefs are one while their laws differ. And Allah knows best.