I want to know what is the law for Nafl (obligatory) fasts on the day of Jumma.
الجواب حامدا و مصلیا
Because of its greatness and for showing reverence to the day of Jumma, it is Makrooh Tanzihi (disliked) to fast on the day of Jumma only. The prohibition of specifying this day for fasting has also been mentioned in Hadith. However, there is nothing wrong with occasionally fasting on the day of Jumma, in fact it is undoubtedly permissible.
And Allah knows best.