Ibn Jurayj (may Allah have mercy on him) says: I said to Ata (may Allah have mercy on him): A man performed Hajj for the first time, and the people made a mistake about the Day of Sacrifice (yawm al-nahr); will it be valid for him? He said: Yes, by my life! It will be valid for him. He said: I think he said that the Prophet (peace and blessings be upon him) said: "Your day of Eid al-Fitr is the day on which you break the fast, and your day of Eid al-Adha is when you sacrifice." And I think he (peace and blessings be upon him) said: "And the Day of Arafah is when the people stand."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9829