The Ramadan crescent moon was not sighted in Saudi Arabia on Tuesday evening, meaning the holy month will officially begin on Thursday instead of Wednesday.

Muslims follow a lunar calendar consisting of 12 months in a year of 354 or 355 days. Sighting a crescent moon heralds the start of Ramadan, the ninth month of the Islamic calendar.

The starting date of the dawn-dusk fasting month is determined by both lunar calculations and physical sightings of a new moon.

Ramadan is observed by more than 1.8 billion Muslims worldwide and is considered a month of fasting and spirituality. It is believed to be the month when the Quran was revealed to the Prophet Mohammed.

Fasting during Ramadan is one of the five pillars of Islam and is mandatory for all healthy Muslims. However, young children, sick individuals, travelers, and women who are pregnant, nursing, or menstruating are exempt.

Muslims have followed the tradition of looking for the crescent moon for thousands of years to determine the start of Ramadan as well as the two major holidays in Islam, Eid al-Fitr and Eid al-Adha.