Eid al-Adha (the Feast of Sacrifice), is the tenth day of the month of Dhu’l-Hijjah. This is the greater and better of the two feasts, which comes after the completion of the Hajj (the pilgrimage), for when the Muslims complete their Hajj (the pilgrimage) they are forgiven.
Rather Hajj (the pilgrimage) is completed on the Day of ‘Arafah [the 9th of Dhu’l-Hijjah] with the standing in ‘Arafah, which is the major pillar of Hajj, as the Prophet (peace and blessings of Allah be upon him) said: “Hajj is ‘Arafah.” Narrated by al-Tirmidhi.
The day of ‘Arafah is the day of ransom from the Fire, when Allah ransoms from Hellfire those who stood at ‘Arafah and Muslims who did not stand at ‘Arafah. Hence the day that follows it is a festival for all Muslims in all regions, those who attended Hajj and those who did not.
It is prescribed for all of them to draw closer to Allah by means of the ritual of shedding the sacrificial blood. (Slaughtering a cattle-beast, and distributing a portion of it amongst the poor)
The virtues of this day may be summed up as follows:
1- It is the best of days before Allah.
Ibn al-Qayyim (may Allah have mercy on him) said in Zaad al-Ma’aad (1/54): The best of days before Allah is the Day of Sacrifice, which is the greatest day of Hajj as it says in Sunan Abi Dawood (1765), where it is narrated that the Prophet (peace and blessings of Allah be upon him) said: “The greatest of days before Allah is the Day of Sacrifice.” Saheeh Abi Dawood
2- It is the greatest day of Hajj.
It was narrated that Ibn ‘Umar (may Allah be pleased with him) said: The Prophet (peace and blessings of Allah be upon him) stood between the Jamaraat on the Day of Sacrifice during his Hajj and said, “This is the greatest day of Hajj.” Narrated by al-Bukhaari, 1742.
3- It is the Eid (holiday) of the Muslims
The Prophet (peace and blessings of Allah be upon him) said: “The day of ‘Arafah, the day of Sacrifice, and the days of al-Tashreeq are our festival, us Muslims, and they are days of eating and drinking.” Narrated by al-Tirmidhi, 773
Courtesy of Islam Q&A