Muslims in South East Asian countries like Indonesia and Malaysia - and much of the Middle East including Egypt, Iraq and Saudi Arabia - will fast on Monday for the start of the month of Ramadan.
Millions more in India, Pakistan and Iran, will likely be marking the start of the lunar month on Tuesday based on moon sightings there.
Muslims follow a lunar calendar, and a moon-sighting methodology can lead to different countries declaring the start of Ramadan a day or two apart.