its forbiden to fast on friday(only)
but can we fast if we are keeping obligatory missed fasts ?
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It is reported in the Sunnah that it is forbidden (or disliked) to fast on Friday alone; Abu Hurayrah, may Allah be pleased with him, narrated that the Prophet said: ''Also, do not exclusively choose Friday as a day of fasting unless it occurs on a day that one regularly fasts.” [Muslim] The Prophet also said: “One amongst you should not fast on Friday unless he fasts on it together with one day before it or one day after it.”
With regard to whether this is in regard to supererogatory fasting only or that it includes the obligatory fasting as well, like a person who makes up the fasting of Ramadan or fasts for an oath, then some scholars stated that it is disliked to fast on Friday as supererogatory fasting but not as an obligatory fasting. Therefore, the forbiddance [or the dislikability] in regard to fasting is to fast Friday as a non-obligatory day, so fasting it for an obligatory fasting is not disliked. However, some other scholars stated that the forbiddance [or the dislikability] includes fasting on Friday as an obligatory fasting as well.
To conclude, if possible, it is better in order to be on the safe side not to fast on Friday even if one is making up an obligatory fasting, but if he needs to fast on this day, he may do so and this is not forbidden or disliked for him.
For more benefit, please refer to Fatwa 87228.
Allah Knows best.