The Ramadan 1439 Hijriyah (AH) is marked with lunar conjunction on Tuesday, 15 May 2018 at 11:47 UT. It falls into Islamic Lunation Number: 17,256.
At the evening of that Tuesday, the hilaal or young crescent moon of Ramadan would be impossible to be seen by naked eyes from almost every place on the globe. On the next evening, however, the hilaal will be easily visible from most part of the world. Therefore, based on global moon sighting possibility, 1 Ramadan 1439 AH will start on Thursday, 17 May 2018.
Crescent Moon Visibility Map for Ramadan 1439 AH
Below is a map showing where and when the crescent moon of Ramadan 1439 AH could be seen. On the evening of Wednesday, 16 May 2018, the hilaal of Ramadan 1439 AH should be easily sighted all over the world.
There is a small probability that people in the Pacific Ocean, like French Polynesia, could sight the moon a day earlier thus moving the start of the Ramadan 1439 AH forward one day.