Ramadan is the ninth month in the Islamic year. Fasting, the third "pillar" or religious obligation of Islam, is required during the daylight hours of all Muslims who are able. Sick people and some travelers in certain conditions are exempted from the fast but must make it up as they are able.
The Islamic calendar is a lunar calendar, and months begin when the first crescent of a new moon is sighted. While many Muslim sects insist on the physical sighting of the moon, there is no such requirement in the Quran, and some Muslim allow the start of the month can be determined by astronomical calculations. Because the Islamic calendar has no correction for the fact that the lunar year is 11 to 12 days shorter than the solar year, Ramadan migrates throughout the seasons.
The daily period of fasting starts at the breaking of dawn and ends at the setting of the sun. In between -- that is, during the daylight hours -- Muslims totally abstain from food, drink, smoking, and sex. The usual practice is to have a pre-fast meal (suhoor) before dawn and a post-fast meal (iftar) after sunset.
During the month, Muslims try to read as much of the Qur'an as they can. Some spend part of their day listening to the recitation of the Quran in a mosque or meeting for Quranic studies or for congregational prayers.
The last ten days of Ramadan are a time of even greater devotion; some Muslims spend the entire time in a mosque. The night on which the Quran was revealed to the Prophet, known as the Night of Power (Lailat ul-Qadr), is generally taken to be the 27th night of the month. Many Muslims spend that entire night in prayer.
The celebration of 'Eid-ul-Fitr, the feast at the end of the month to break the fast, is a traditional practice rather than a religious one.
Non-Muslims are sometimes sensitive about not giving offence during Ramadan -- for example, by conducting military operations. Numerous examples indicate that this sensitivity is unnecessary: Mohammed himself fought during Ramadan in 624. In 1973, Egypt and Syria attacked Israel during Ramadan, which happened that year to coincide with the Jewish Yom Kippur. In 1982, Iran launched an attack on Iraq that they explicitly called "Operation Ramadan." Muslims have rarely shown any reluctance to wage war during Ramadan.