The Islamic holy month of Ramadan will begin Thursday in Saudi Arabia, the land of the religion's two holiest sites, authorities said.
"The first day of the holy fasting month of Ramadan falls on Thursday corresponding to May 17, 2018," the Royal Court said in a statement published by state news agency SPA.
The start date is set by both lunar calculations and physical sightings which determine when the last day of one lunar month ends and a new one begins.
Traditionally, many Muslim-majority countries have followed the dates set by Saudi religious authorities, but in recent years many have used their own astronomical calculations.
Ramadan is a holy month for the world's nearly 1.5 billion Muslims, many of whom practise the ritual of dawn-to-dusk fasting and prayers.
Tradition holds that it was during Ramadan that the Prophet Mohammed started receiving revelations of the Muslim holy book, the Koran.
Ramadan is one of the five "pillars" of Islam.
The others are the profession of faith ("there is no God but God and Mohammed is his messenger"), the obligation to pray five times a day, charity, and the pilgrimage to Mecca.