The Days during which fasting is prohibited

Question :

What are the days during which it is disliked to fast?

Answer:

The days during which fasting is prohibited are Friday, as it is not permitted to single out Friday for fasting voluntary fasts, because the Messenger prohibited that. Likewise, Saturday should not be singled out for voluntary fasting. However, if the person fasts Friday and Saturday along with it or Thursday along with it, there is no harm in that, as the Hadiths from the Messenger of Allâh have stated. Likewise, it is prohibited to fast the day of 'Eidul-Fitr and that is (strictly) forbidden. Similarly, fasting 'Eidun-Nahr and the days of At- Tashreeq should not be fasted, because the Messenger prohibited that. However, concerning the days of At-Tashreeq, a report has come that proves the permissibility of fasting it to make up for the sacrifice of Hajjut-Tamattu' and Hajjul-Qiran specifically for whoever was not able to sacrifice. This is based on what is confirmed in Al-Bukhari from 'A'ishah and Ibn 'Umar. They both said:
"It was not permitted to fast the days of At-Tashreeq except for whoever could not make the sacrifice."

In reference to these days being fasted for voluntary fasting or other reasons, then that is not permissible just like the day of the 'Eid. Likewise, it is not permissible to fast the 30th day of Sha'ban if the sighting of the new moon has not been confirmed, for verily, it is a day of doubt. It is not permissible to fast it according to the most correct view of the two opinions of the scholars. It makes no difference whether it is clear outside or cloudy due to the authentic Hadiths that prove the prohibition of that. And Allâh is the Giver of success.

Source:
Ash-Shaykh Ibn Baz
Fatawa Islamiyah, Vol. 3 Pages 340-342