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.
Traditionally, countries announce if their moon-sighting council spots the Ramadan crescent the evening before fasting begins.
Across the world, Muslims fast each day for the entire month of Ramadan, abstaining from food and drink from dawn to dusk. That means around 15 hours without food, water, cigarettes or caffeine.
Fasting is aimed at drawing worshippers closer to God through self control, remembrance and humility. The challenge of fasting for many is also a chance to reset spiritually and physically, kick bad habits and purify the heart.
During the day, Muslims must also abstain from sex, gossip and cursing, and are encouraged to focus on meditative acts like prayer, reading the Koran and charity.
It's common practice across many Muslim-majority nations for liquor stores and hotels to curb the sale of alcohol during Ramadan. Often, restaurants shutter their doors during the day.
Those exempt from fasting include children, the elderly, the sick, women who are pregnant, nursing or menstruating, and people travelling.
The Ramadan fast begins with a pre-dawn meal called "suhoor" to prepare hungry stomachs for the long day ahead. A typical suhoor often includes bread, vegetables, fruits, yoghurt, tea, as well as lentils and beans.
At sunset, when it's time to mark the end of the daylong fast, families and friends gather for an evening meal known as "iftar".
