Ramadan in 2018 will start approximately on Wednesday, the 16th of May (16/5/2018) and will continue for 30 days until Thursday, the 14th of June.
Note that in the Muslim calendar, a holiday begins on the sunset of the previous day, so observing Muslims will celebrate Ramadan on the sunset of Tuesday, the 15th of May.
Although Ramadan is always on the same day of the Islamic calendar, the date on the Gregorian calendar varies from year to year, since the Gregorian calendar is a solar calendar and the Islamic calendar is a lunar calendar. This difference means Ramadan moves in the Gregorian calendar approximately 11 days every year. The date of Ramadan may also vary from country to country depending on whether the moon has been sighted or not.