Eid al-Adha for Muslims is a four day festival that starts on the 10th day of the month of Dhu al-Hijjah 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.