In most communities in the United States, Eid begins at sundown on Monday, May 2, and lasts one to three days, depending on cultural tradition.
Islam follows a lunar calendar, with the sighting of the crescent moon determining the start and end of each month. This means the date of Eid, which marks the beginning of the Islamic month of Shawwal, changes in the Gregorian calendar each year.
Eid al-Fitr, which means "festival of breaking the fast," comes after a month of abstaining from food and drink from sunrise to sunset in observance of Ramadan.
MORE: How Muslims observe Ramadan, holiest month in Islamic calendar
To celebrate, many Muslims attend communal prayers, listen to a khutuba (sermon), and give Zakat al-Fitr (charity in the form of food). It's an occasion for Muslims to show their gratitude to God and give alms to the poor.
Many Muslims will dress in their finest clothes, exchange small gifts and host parties.
"Everyone's just so excited to be gathering together and looking good and feeling good," said Aisha Rawji, founder of KYNAH, a Los Angeles-based retailer that sells Indian designer clothing.
The estimated 3 to 4 million in the U.S. who celebrate, from African Americans to South Asian immigrants and many more, bring their own social and cultural traditions to the holiday, according to National Geographic.
Some Muslim girls and women will get intricate henna tattoos painted on their hands, and others will let kids pop balloons with money or candy inside.
Non-Muslims can greet their celebrating friends and neighbors with the phrase, "Eid Mubarak," which means "blessed Eid" or "blessed holiday" in Arabic.