Turkey - along with Muslim communities in Europe, the US, Canada and Australia - previously declared that Saturday will be the first day of Ramadan, based on astronomical calculations of the new moon birth.
Saudi Arabia's High Judicial Court announced that the moon was not sighted on Thursday evening, and that it will convene on Friday evening for moon sighting again.
This is unprecedented, as a lunar month can only be 29 or 30 days, and according to the Saudi Umm al-Qura calendar cited by the court, Thursday was the 29th of Shaban, the last lunar month before Ramadan.
According to astronomers, the new Ramadan moon will be visible on Friday around the world, either directly with the naked eye or with a telescope.
In Yemen, rival parties have both announced Saturday as Ramadan's first day.