Scientifically, Ramadan's new moon should be visible on Friday evening, confirming Saturday as the first day of fasting.
As per astronomical calculations, Ramadan fasting from May 27 to June 24.
The Muslim holy month of Ramadan will begin either on the eve of Saturday May 27 or on Sunday May 28, depending on moon sighting on Friday.
Lebanon, Iraq, Yemen, and Turkey have already confirmed Saturday as the first day of Ramadan.
Saudi Arabia, Qatar, Kuwait and the UAE had called for moon sighting on Thursday evening, May 25, but unable to sight the moon they will reconvene on Friday to announce Ramadan's start date.
Egypt, Jordan, Oman, South Africa, Nigeria, Pakistan expect Ramadan on the eve of Saturday May 27 or Sunday May 28, depending on moon sighting on Friday.
The beginning of Shaban, the last lunar month before Ramadan, differed by country. For some the first day was April 27, for others it was April 28.
Muslim lunar months last between 29 and 30 days, depending on sighting of the moon on the 29th night of each month. If the moon is not visible, the month will last 30 days.
In order to declare the beginning of Ramadan, Saudi Arabia and other Muslim-majority countries depend on the testimonies of local moon sighters. The Judicial High Court then makes a decision on when Ramadan begins.
By the Gregorian solar calendar, Ramadan comes 10 to 12 days earlier each year. Last year, the first day of Ramadan was on June 6, 2016.
In the United States, the Fiqh Council of North America (FCNA) announced that Ramadan would be observed from May 27, based on astronomical predictions.
Muslim communities in the UK, Europe and Australia will also observe Ramadan starting from May 27.
Astronomically the birth of a new moon can be calculated, but the actual visibility of the crescent depends on factors such as atmospheric conditions, cloudiness, and the distance between the sun and the moon on the horizon.
Calculation indicates that the moon should be visible from most countries in the evening of May 26, either with the naked eye or a telescope.
For Muslims, Ramadan is the month in which the first verses of the Quran, Islam's holy book, were revealed to the Prophet Muhammad more than 1,400 years ago.
During Ramadan, Muslims abstain from eating, drinking, smoking and sex from dawn to sunset. This fasting is intended to bring the faithful closer to God and to remind them of the suffering of those less fortunate.
Ramadan fasting is one of the five pillars of Islam, along with the Muslim declaration of faith, daily prayer, charity and performing the Hajj pilgrimage to Mecca.
In Muslim-majority countries, offices are required by law to reduce working hours, and many restaurants are closed during daylight hours.
"Ramadan Mubarak" and "Ramadan Kareem" are common greetings exchanged in this period, wishing the recipient a "blessed" and generous Ramadan.
Last year, fasting hours across the world ranged between 11 and 22 hours. This year, fasting hours will range between 10 hours in Chile and 21 hours in Greenland.
At the end of Ramadan, after 29 or 30 days, Muslims celebrate the Eid al-Fitrholiday. Eid al-Fitr in Arabic literally means "festival of breaking the fast".
Depending on the actual start date of Ramadan and moon sighting on the 29th night of Ramadan, the Eid al-Fitr this year will fall between Sunday, June 25 and Tuesday, June 27.