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(CNN) -- Everything you need to know about Ramadan, the holiest month in the Muslim calendar.
What is Ramadan?
Ramadan is the name of the ninth month in the Hijri, or Islamic calendar. The word derives from the Arabic ramida or ar-ramad meaning a fierce, burning heat.
How important is it?
Ramadan is the most sacred month in the Muslim year, commemorating the revelation of the Holy Qur'an -- the sacred religious text of Islam -- by the angel Gabriel to the Prophet Muhammad.
What does it involve?
The main obligation of the festival is the Sawm, or fast. Believers are expected to refrain from eating and drinking from dawn (fajr) until dusk (maghrib) for the entire month, a discipline that is thought to burn away all sins (hence the origin of the word 'ramadan'). The Sawm is considered one of the five "pillars", or foundations of Islam, the others being the Shahadah (profession of faith), Salah (praying five times daily), Zakah (charity) and Hajj (pilgrimage to Mecca).
When does the festival start?
Ramadan begins with the sighting of the new moon. Because it is calculated according to a lunar rather than a fixed calendar, its precise dates change from year to year, and it can begin at different times in different parts of the world. According to Saudi authorities, Ramadan started this year on September 24, and ends on October 23.
And is it just about fasting?
No. As well as eating and drinking, the faithful are expected to abstain from smoking and sexual relations between dawn and dusk, and to abjure lies, slander, greed, covetousness, giving false oath and denouncing someone behind their back (all of these are proscribed throughout the year by Islam. To commit them during Ramadan, however, is considered particularly sinful). Muslims are also expected to recite a special 'night prayer', the taraweeh, in addition to the five daily prayers.
Is every Muslim expected to fast?
No. Young children -- before the onset of puberty -- are exempted, as are those with an illness or medical condition that would be exacerbated by fasting. If the medical condition is only temporary, the sufferer is required to make up for the days missed once they have recovered. If the condition is permanent the spiritual benefits of fasting can be obtained by feeding a needy person for a month.
And what happens if you break the fast?
If a believer intentionally breaks the fast, or performs any other prohibited activity, they become subject to a penalty, or kaffara (literally, atonement). This can take the form of an extra 60 days of fasting at the end of Ramadan, feeding 60 people in need, or -- not quite so easy in the modern world -- freeing a slave.
And what happens at the end of Ramadan?
Ramadan officially ends on the first day of the month of Shawwal. This heralds a three-day celebration called Eid al-Fitr, or the "Festival of Breaking Fast", a joyous occasion during which believers attend mosques, give gifts, visit friends and family and decorate their homes.
Worshippers pray at a mosque in Manila at the start of Ramadan.