What is the ruling on fasting the eleventh and twelfth days of Dhul-Hijjah?

The Days of Tashreeq (the eleventh, twelfth, and thirteenth days of Dhul-Hijjah) are days of eating, drinking, and remembrance of Allah. The Prophet, peace and blessings be upon him, forbade fasting on these days, and he only permitted fasting them for the Mutamatti' (one performing Umrah then Hajj) or the Qaarin (one combining Umrah and Hajj) who could not find a sacrificial animal. The majority of scholars are of the opinion that voluntary fasting on these days is not valid, and the correct view is that it is impermissible to fast them as a make-up for Ramadan.

So, whoever fasts the Days of Tashreeq or some of them, without being a Mutamatti' or Qaarin who could not find a sacrificial animal, must seek Allah's forgiveness. And if one fasts them as a make-up for missed days of Ramadan, that will not suffice, and they must make them up again.