Observance of Ramadan is also mandated in the Quran's second chapter, verses 183-185:
"O you who have believed, decreed upon you is fasting as it was decreed upon those before you that you may become righteous ... The month of Ramadhan [is that] in which was revealed the Quran, a guidance for the people and clear proofs of guidance and criterion. So whoever sights [the new moon of] the month, let him fast it."
Ramadan comes at a different time every year because it is the ninth month of the Islamic calendar, a date-keeping system based on lunar cycles, unlike the Gregorian calendar (the one used by most of the world, including the US), which is based on the solar year. A new month begins with the appearance of the new moon, or the crescent moon, and ends with the next appearance of a new moon.
The month of Ramadan thus moves backwards about 10 days every year relative to the Gregorian calendar.
The conclusion of Ramadan is marked with a major celebration known as Eid ul-Fitr (or Eid al-Fitr), the Feast of Fast-Breaking. It starts the day after Ramadan ends and lasts for three days.
Eid al-Fitr includes special prayers and meals with friends and relatives, and gifts are often exchanged.
This year Ramadan begins on Friday, May 26 and ends on Saturday, June 24.