Ramadan opens the first day of the ninth month of the Hijri calendar, and this rule stores its length as twenty nine days specifically so the month is never reported as running into the first of Shawwal, since the month is actually twenty nine or thirty days depending on the sighted or calculated end, a detail worth flagging rather than papering over. The obligation to fast during the month rests on a direct Quranic command, sura two, verses one hundred eighty three to one hundred eighty five, which frames the fast as prescribed for earlier communities as well and ties the month specifically to the beginning of the Quran's own revelation to Muhammad.
Within the month, tradition holds that one night, most often sought among the last ten and especially the odd numbered nights, is Laylat al Qadr, the Night of Power, described in the Quran's ninety seventh sura as better than a thousand months, the night the revelation began. The fast itself runs from the onset of dawn to sunset, and the discipline is textual rather than merely customary, resting on the same verses that name endurance in fasting as a means toward taqwa, God consciousness, the stated purpose the Quran gives for the obligation.