Eid al-Adha
The festival is to celebrate Ibrahim's (Abraham) willingness to sacrifice his son, Isma’il, because Allah told him to do so in a dream. Showing that he loved Allah more than his own son. And at the final moment, after taking his son's consent, as he was swinging his axe to kill his son God told an angel  to switch a lamb instead of his son, and this act of sacrificing a lamb is copied by Muslims all over the world on Eid.