Muslims attend an Eid al-Adha mass prayer marking the end of the annual Hajj pilgrimage, in Mombasa, Kenya Sept. 12, 2016. ISIS recently declared that the major Muslim holiday should begin on Tuesday as opposed to Monday. REUTERS/Joseph Okanga

The Islamic State group has declared Tuesday the first official day of the sacred Muslim holiday Eid al-Adha. The militants also known as ISIS threatened to “flog” and imprison anyone who chose to violate their decision, a local source told Iraqi News.

ISIS reportedly released a statement to civilians in Iraqi markets and neighborhoods that stated Tuesday would be the official start date of the four-day-long Islamic holiday. Many were reportedly angered by the decision as many Arab and Islamic countries declared Monday the first official day of the holiday this year.

Residents in ISIS' territories, including, Mosul, a city located in northern Iraq, were among those who received the warning from ISIS. Mosul has been occupied by the Islamic State since 2014.

“The timing that ISIS declared for Eid al-Adha sparked anger among Mosul’s residents, and they have decided to reject and not to follow the instructions imposed by the terrorist organization,” the source told Iraqi News.

Edi al-Adha is a holy holiday that commemorates the biblical story of Abraham’s sacrifice to God. Livestock are typically slaughtered by those who can afford to purchase the animals and the meat is distributed to the poor.

The holiday marks the end of another major Muslim event, hajj, where Muslims make a holy pilgrimage to Mecca in Saudi Arabia. This year's event began Saturday, and saw Saudi authorities direct 60,000 security officers around hajj sites to help prevent potential attacks from ISIS supporters, among other threats.

Deemed the holiest of the Eid holidays, Eid al-Adha is celebrated by millions of Muslims around the world. The start date changes every year because it is based on the lunar calendar.