Imam Al Uthaymeen mentioned:
“The first day is Eid, there is no confusion in regards to that. The second and the third day are also Eid due to the statement of the prophet صلى الله عليه و سلم leave them – meaning the two young girls whom were singing – for indeed it is the days of Eid.
The word days is plural and the least a plural can be is three. Thus there is no problem that a person makes the three days all of it Eid except in one matter, and it is fasting. This is since fasting, if he said he won’t fast the second and third day of Shawaal because fasting it is prohibited like fasting Eid, then this is not permissible.
As for in Dhul Hijjah, then it is known that it is not permissible to fast the days of Tashreeq. Thus if we added three days to the day of Eid, it becomes four.
Perhaps you intend that which the people do from celebrations. Then we say it is three days in both Eids, Eid Al Adhaa and Eid Al Fitr”
Majid Jawed Al-Afghanee