Eid Al Adha, the festival of sacrifice, is one of the two main sacred holidays in Islam, the other one is Eid al-Fitr, the marking of the end of fasting during the holy month of Ramadan.
Eid itself means holiday or festive, and Al Adha comes from the known story of the attempt of Prophet Ibrahim (Abram) sacrificing his son Ismail (Ishmael) to the Lord upon instructions from the Lord then he was asked to sacrifice the Ram instead ending the human sacrificing which was a custom in the old civilizations.
Muslims perform the Hajj - Pilgrimage to Mecca in western Arabia where the story of Ibrahim's sacrificing took place, and they mimic the acts of Ibrahim, his wife Hagar, and their son Ismail who settled there and built the first place to worship God, the Kaaba, the black square-cubic shaped building in the now thriving city.
The story is long and very interesting, I suggest you look it up from trusted sources and cross-examine it by a couple of sources so you won't end up learning a beautiful story from hating bigots.