In 'Umdat al-Qaari 10/38, al-'Aynee said, "The scholars differed into a number of opinions concerning the greatest day of hajj (yawm al-hajj al-akbar).
The first opinion is that it is the day of slaughter (the tenth of Thul Hijjah). This was the opinion of 'Ali ibn Abi Taalib (may Allah be pleased with him), 'Abdullah ibn Abi Awfa, ash-Sha'bi, and Mujaahid.
The second opinion is that it is the day of 'Arafah. This was transmitted from 'Umar and his son 'Abdullah ibn 'Umar.
The third opinion is that all the days of hajj are yawm al-hajj al-akbar. This is because a period of time may be referred to as a day like the day of Bu'aathm the day of the camel, and the day of Siffin (this is reference to battles which took place over a number of days). This was the opinion of Sufyaan ath-Thawri.
يوم الحج الأكبر