Ramadan is the ninth month of the Islamic calendar, and the month in which the Quran was revealed. Fasting during the month of Ramadan is one of the Five Pillars of Islam. The month is spent by Muslims fasting during the daylight hours from dawn to sunset.The Quran was sent down to the lowest heaven during this month, thus being prepared for gradual revelation by Jibreel (Gabriel) to the Islamic prophet Muhammad. Furthermore, Muhammad told his followers that the gates of Heaven would be open for the entire month and the gates of Hell (Jahannam) would be closed. The first day of the next month, Shawwal, is spent in celebration and is observed as the "Festival of Breaking Fast" or Eid al-Fitr. The Islamic calendar is a lunar calendar, and months begin when the first crescent of a new moon is sighted. Since the Islamic lunar calendar year is 10 to 11 days shorter than the solar year and contains no intercalation, Ramadan migrates throughout the seasons. The Islamic day starts after sunset. 

Source:Wikipedia