Assalamu Alaikum Wa Rahmatullah
I think I may have kept one of my qadha Ramadan fasts on the 13th Dhul hijjah (third day after eid ul adha). Is my fast valid or do I have to repeat it?
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
It is disliked (makruh) to fast on the two Eids, and on the 11th, 12th and 13th of Dhul Hijjah (Ayyamut Tashreeq). However, if someone fasts on these days such as making up a qadha fast, it will be valid. Similarly, if someone starts a fast on these days and thereafter breaks it, then there’s no qadha upon him for the fast which he broke on these days. 
And Allah knows best
Answered by Aalimah Sabrina Saidova
Checked and approved Mufti Mohammed Tosir Miah
Darul Ifta Birmingham