It is forbidden to fast on the first day of Eid al-Fitr, and on all four days of Eid al-Adha. This is because those days are the days to eat, drink, share and celebrate the blessings that God has granted us. There are several hadiths related to this prohibition:
A’isha (Allah be pleased with her) said that:
“The Prophet (ﷺ) forbade to observe fast on two days: the day of Fitr and the day of Adha.” (Muslim asl-Siyam 22)
Nubaisha al-Hudhali reported Allah’s Messenger (ﷺ) as saying:
“The days of Tashriq are the days of eating and drinking.” (Muslim, Sawm, 23, 1141)
Uqbah bin Amir narrated that The Messenger of Allah said:
“The Day of Arafah, the Day of Nahr, and the Days of Tashriq are Eid for us. The people of Islam and they are days of eating and drinking” (Tirmidhi, Sawm, 59, 773; Nasa, Hajj, 1, 2619; Abi Dawood, Siyam, 815, 2419)
The days of tashriq are the 11th, 12th, 13th of the month Dhi’l Hijjah, which are the 2nd, 3rd and 4th days of Eid al-Adha.