The Islamic calendar is a lunar calendar and its lunar month is the time between successive new crescent moons (29 to 30 days). That makes their 12 month lunar year a total of 354 or 355 days or 11 days shorter than the seasonal year on which our Gregorian calendar is based.
Ramadan is the ninth month of the Islamic calendar, and is observed by Muslims worldwide as a month of fasting to commemorate the first revelation of the Quran to Muhammad according to Islamic belief.
The month lasts 29–30 days based on the visual sightings of the crescent moon. If it isn’t seen on the evening of June 17, the start of Ramadan will wait until the evening of June 18. The start date for Ramadan in 2015 will vary with location. South America might catch the thin crescent moon on June 17. The evening of June 18 might mark the date for much of the rest of the world. Ramadan continues until the appearance of next month’s young crescent moon.