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The 10th of Dhul-Hijjah, commemorating Ibrahim's sacrifice.
Eid al-Adha falls on the 10th of Dhul-Hijjah, the Day of Nahr during Hajj, and commemorates Ibrahim's willingness to sacrifice his son in obedience to Allah, and Allah's mercy in ransoming him with a sacrificial animal instead (Qur'an 37:102–107). It is the greater of the two Eids in many scholars' view, coinciding with the culmination of Hajj for those on pilgrimage.
Like Eid al-Fitr, fasting on this day is forbidden (Bukhari 1990). Its central additional rite is the udhiyah (sacrifice), offered by those who are able, in remembrance of Ibrahim's submission and as an act of worship and charity combined.
Qur'an
Reference: Qur'an 37:102–107
So when he had reached the age to work with him, Ibrahim said: O my son, indeed I have seen in a dream that I should sacrifice you... And We ransomed him with a great sacrifice.
Hadith
Sahih al-Bukhari · 1990
Sahih
“These are two days on which the Messenger of Allah ﷺ forbade fasting: the day you break your fast (of Ramadan), and the day on which you eat from your sacrifice.”
Educational overview — not a fatwa. Where schools differ (such as the extra-takbir count), both positions are presented; follow your imam or a qualified local scholar.