Ramadan is the ninth month of the Islamic calendar; Muslims worldwide observe this as a month of fasting. This annual observance is regarded as one of the Five Pillars of Islam. The month lasts 29–30 days based on the visual sightings of the crescent moon, according to numerous biographical accounts compiled in the hadiths. The word Ramadan comes from the Arabic root ramiḍa or ar-ramaḍ, which means scorching heat or dryness. Fasting is fard ("obligatory") for adult Muslims, except those who are ill, traveling, pregnant, breastfeeding, diabetic or going through menstrual bleeding.