Reference: Fataawa Ramadhaan – Volume 2, Pages 657-658, Fatwa No.663
Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Fatwa No.11810
Question: Is it permissible to make up missed fasts of Ramadhaan on Thursday and Friday or not?
Response: Whoever missed any fasts of Ramadhaan, then it is permissible to make up [the missed] fasts on both Thursday and Friday and likewise offer voluntary fasts [on both these days]. However, that which is prohibited is to single out Friday by fasting a voluntary fast.
And with Allaah lies all the success, and may Allaah send prayers and salutations upon our Prophet Muhammad ﷺ and his family and his companions.