The Muslim holy month of Ramadan will begin either on Saturday, May 27 or on Sunday, May 28, depending on moon sightingon the eve of May 27,2017
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.
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 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.
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 Europe will also observe Ramadan starting from May 27, as per the European Council of Fatwa and Research and the Islamic Relief UK charity.
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.