Ramadan is the month in the Islamic lunar calendar.
Fasting during Ramadan is one of the five pillars of Islam.
The first and last dates of Ramadan are determined by the lunar Islamic calendar.
The Islamic lunar calendar is based on sighting the moon by the human eye.
Muslim lunar months last 29 or 30 days. If the new moon is not visible on the twenty-ninth after sunset, then the month lasts 30 days.
The actual visibility of the crescent depends on factors such as atmospheric conditions, cloudiness and the distance between the sun and the moon.