بسم الله الرحمن الرحيم
Q: What is the ruling concerning he who fasts the day of the Eid despite the fact that he knows it’s the Eid?
A: Fasting the day of the Eid is not permissible due to that which is firmly established upon the Prophet -sallahu alayhi wa salaam- from authentic narrations regarding the prohibition of fasting on the two days of the Eid. Thus the scholars have consensus on its prohibition and whoever does so then binding upon him is repentance and to never return to such an action.
And with Allah is success, and may Allah’s praise and peace be upon our Prophet Muhammad and his family, and companions.
Source: http://www.alifta.net/Fatawa/FatawaChapters.aspx?languagename=ar&View=Page&PageID=67&PageNo=1&BookID=12
Translated by Najeeb ibn Yusuf Al Anjelesi