Every year, the world’s 1.7 billion Muslims—roughly a quarter of the global population—celebrate the Islamic holy month of Ramadan, a time for fasting and extra piety to cleanse the soul and come closer to God. Ramadan is a major global event, but just when, exactly, it begins is a matter of some debate. The month-long holiday is observed principally in Muslim-majority countries in Africa and Asia but also by the believers around the world, including the roughly 3.3 million Muslims living in the U.S.