File photo: Displaced families celebrate with their children the Eid al-Fitr Muslim holiday, which marks the end of the fasting month of Ramadan, at a luna park in Afrin, Aleppo province, Syria. AFP
In some countries with a Shia Muslim majority, authorities have said the holiday will start on Saturday.
"Tomorrow, Friday, is the first day of Eid al-Fitr for this year," with Thursday the last day of the holy month of Ramadan, the official Saudi Press Agency said on its Twitter account, citing a royal court statement.
The timing of Eid al-Fitr is determined by the sighting of the crescent moon, in accordance with the Muslim lunar calendar.
The holiday is normally celebrated with family gatherings.
Authorities in Egypt, the United Arab Emirates, Qatar, Jordan and Sudan also announced Eid al-Fitr will start on Friday.
In Lebanon, Sunni clerics said the holiday will begin on Friday, while some Shia leaders announced a Saturday start to the holiday.
Libya, ruled by two rival administrations, will mark Eid on Friday in the country's east and on Saturday in areas under the control of the Tripoli-based government.
Statements from Iran's supreme leader Ayatollah Ali Khamenei and Iraq's highest Shia authority, Grand Ayatollah Ali Sistani, said Eid would start on Saturday in their countries.
In Oman, too, the holiday will start on Saturday.
The daytime fasting month of Ramadan is one of the five pillars of Islam.
Observant Muslims refrain from eating and drinking from dawn to dusk, and traditionally gather with family and friends to break their fast in the evening.
It is also a time for prayer, with the faithful converging in large numbers on mosques, especially at night.
Saudis are expected to observe a four-day holiday for Eid al-Fitr.