To determine when exactly the holy month will begin, Muslim-majority countries look to local moon sighters, according to Al Jazeera.
The lunar months last between 29 and 30 days, depending on the sighting of the moon on the 29th night of each month. If the moon is not visible, the month will last 30 days.
What do Muslims do during Ramadan and why?
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Muslim women gather for a special Eid ul-Fitr morning prayer at the Los Angeles Convention Center in Los Angeles, California.
Ramadan is known as the holy month of fasting, with Muslims abstaining from eating and drinking from sunrise to sunset.
Fasting during the holiday is one of the Five Pillars of Islam, along with the daily prayer, declaration of faith, charity and performing the Hajj pilgrimage in Mecca, Saudi Arabia.
In 2016, according to Al Jazeera, fasting hours around the globe ranged between 11 and 22 hours and in the US, 16 to 18 hours.
The fast is intended to remind Muslims of the suffering of those less fortunate and bring believers closer to God (Allah, in Arabic).
During the month, Muslims also abstain from habits such as smoking, caffeine, sex and gossip; this is seen as a way to both physically and spiritually purify oneself while practicing self-restraint.
Here’s what a day of fasting during Ramadan is like:
Muslims have a predawn meal called the “suhoor.”
Then, they fast all day until sunset.
At sunset, Muslims break their fast with a sip of water and some dates, the way they believe the Prophet Muhammad broke his fast more than a thousand years ago.
After sunset prayers, they gather at event halls, mosques or at home with family and friends in a large feast called “iftar."
How is the end of Ramadan celebrated?
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A girl blows bubbles during an Eid celebration in London, England.
Toward the end of the month, Muslims celebrate Laylat al-Qadr or “the Night of Power/Destiny” — a day observers believe Allah sent the Angel Gabriel to the Prophet Muhammad to reveal the Quran’s first verses.
On this night, which falls on one of the last 10 nights of Ramadan, Muslims practice intense worship as they pray for answers and seek forgiveness for any sins.
To mark the end of Ramadan, determined by the sighting of the moon on the 29th, a 3-day celebration called Eid al-Fitr brings families and friends together in early morning prayers followed by picnics, feasts and fun.
Does every Muslim fast during Ramadan?
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Seventeen months -old Pakistani Muslim Ali Khaja gives his hat to his grandfather Ahsan Khaja before for the special 2011 Eid ul-Fitr morning prayer in Los Angeles, California.
According to most interpreters of the Quran, children, the elderly, the ill, pregnant women, women who are nursing or menstruating, and travelers are exempt from fasting.
Some interpreters also consider intense hunger and thirst as well as compulsion (someone threatening another to do something) exceptions.
But as an entirety, whether Muslims fast or not often depends on their ethnicity and country.
Many Muslims in Muslim-majority countries, for example, observe the monthlong fast during Ramadan, according to 2012 data from the Pew Research Center.
In fact, in Saudi Arabia, Muslims and non-Muslims can be fined or jailed for eating in public during the day, according to the Associated Press.
But in the United States and in Europe, many Muslims are accepting of non-observers.
Islam is the fastest growing religion in the world. In fact, if current trends continue, Muslims will surpass Christians as the world’s largest religious group in the second half of this century, according to the Pew Research Center.
As of 2010, there were an estimated 1.6 billion Muslims around the world, making up the majority of the population in 49 countries.
Between 2010 and 2015, 31 percent of all babies born around the world were born to Muslims.
Muslims also have the youngest average age of all the major religious groups, Pew researches noted. In 2015, the median age of Muslims around the globe was 24 whereas the median age of non-Muslims was 32.
Those factors coupled together have led to the population projections in the second half of this century.