Eid al-Adha for Muslims is a four day festival that starts on the 10th day of the month of Dhul Hijja commemorating the willingness of Ibrahim's sacrifice of his son Ishmael at Allah's command.
Muslims will go to a sermon or a mosque or outdoor setting and pray. Many will sacrifice an animal known as qurbani to represent Abraham sacrificing his son for Allah. Others may travel to Mecca to perform a Hajj pilgrimage.
The Gregorian date is difficult to determine because Eid al-Adha is determined by the Islamic calendar which is a lunar calendar based on the cycles of the moon.