It is undesirable to fix Jumu'ah for voluntary fast. One can, however, observe fasting if Friday occurs in his routine of fasts, i.e., if one observes fast on alternate days and Jumu'ah occurs on the day when he observes fast, or if one observes the fast of the Day of 'Arafah, or the Day of 'Ashura', and Friday occurs on that day, or if Friday occurs during the Ayyam Al-Beid, or Friday occurs when one is observing fasts of Nadhr (fasts one has vowed for). There is no harm in observing fast on Friday in all such cases but its special observance on Friday is not recommended.