Answered by: Maulana Tahsin Alam
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 valid, despite it being makrūh (disliked). 
Only Allah Ta’ala knows best
Written by Maulana Tahsin Alam
Checked and approved by Ustadha Sabrina
Darul Ifta Birmingham
 Imam Kasani, Badāiʿ al-Ṣanāiʿ, volume 2 pg. 586.