Reference: Fataawa Ramadhaan – Volume 2, Page 724, Fatwa No.732
Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-`Ilmiyyah wal-Iftaa. – Fatwa No.12961
Question: What is the ruling regarding the person who fasts the day of ‘Eed despite knowing it is the day of ‘Eed?
Response: It is not permissible to fast on the day of ‘Eed for that which has been established from the Prophet ﷺ in numerous authentic narrations regarding the prohibition of fasting the two days of ‘Eed – al-Fitr and an-Nahr (al-Adh.haa).
And there is concensus of the scholars of Islaam as to its prohibition, and whoever does that is required to repent to Allaah (Subhaanahu wa Ta’aala) and not repeat it.
And with Allaah lies all the success, and may Allaah send prayers and salutations upon our Prophet Muhammad ﷺ and his family and his companions.