Editor's note: This story was originally published in June 2015. This year, millions of Muslims around the world are marking the start of Ramadan on Monday, June 6.
Thursday is the start of Ramadan, the Muslim holy month marked by daily fasting (no food and no water) from dawn to sunset, increased religious observance and self-reflection.
For 30 days, followers of Islam pray and refrain from smoking, bad behavior — including cursing, gossiping or fighting — and impure thoughts.
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Ramadan ends with Eid al-Fitr, the feast of fast-breaking. It's a three-day holiday that includes special prayers and meals.
What's the history of Ramadan?
Ramadan is celebrated as the month during which the Prophet Muhammad received the initial revelations that make up the Quran, the Muslim holy book.
Why is Ramadan at a different time each year?
Ramadan is the ninth month of the Islamic calendar, and begins with the sighting of the new moon. Because the moon can be spotted at different times in various parts of the world — and sometimes because of methodology — Ramadan's start can vary slightly from country to country.
This year, religious authorities in Saudi Arabia, Egypt, Iran, Indonesia and other parts of the world announced based on their sightings of the moon that daily fasting would begin Thursday.
And since the lunar calendar doesn't quite line up with the solar calendar, Ramadan usually moves up by about 11 days each year — and passes through all seasons.
Why do Muslims have to fast?
Fasting and focusing on prayer and charity during Ramadan is meant to purify the body and spirit and bring the faithful closer to God; fasting during the month is also one of the five pillars of Islam.
This year, Ramadan coincides with the start of summer, meaning followers could be in for 15 hours or more of fasting.
In the Twin Cities, at the beginning of Ramadan, dawn will likely happen around 4:45 a.m.; the sun will set around 9 p.m. over the course of the month.
In Europe, there has been ongoing debate about whether Muslims there should follow Ramadan on local time or on Mecca time. Daylight during Ramadan in some parts of Europe can last 16 hours or more, compared to 12 or 13 in Mecca.
Do all Muslims have to fast during Ramadan?No; usually only those who have reached puberty and are in good health will fast during the holiday.
Pregnant women or those who are nursing, those who are traveling, the elderly and the sick are also usually exempt. Still, that hasn't stopped some diabetic Muslims from following through with the fast.
But remember: Followers of Islam don't fast the entire month. They eat a pre-dawn meal called a "suhoor" and break the fast after sunset each day with a meal known as "iftar," shared with family and friends.
Meanwhile, several schools in London have banned students from fasting during Ramadan over health concerns, unless parents have met with their child's head of school.
A letter that one school sent to parents and posted online says, in part:
"In addition to it being the hottest time of the year, this is also a very busy time of the school year with additional sports activities, sports days and annual year group visits etc. ...Last year on occasion the period between opening and closing fasting was in excess of 18 hours. For a child this is a significant amount of time with sustenance and water.
"... Previously, we have had a number of children who became ill and children who have fainted or been unable to fully access the school curriculum in their attempts to fast."
Will I see people celebrating Ramadan in public?
Typically, the start of the month is welcomed with greetings such as "Ramadan mubarak!"
Many mosques and aid organizations, including the Abu Khadra Masjid in Columbia Heights, Minn., also organize or host free iftar meals for the public.
Those traveling at airports may see observant Muslims engaging in prayer or going through ablution, the cleansing or washing of certain areas of the the body — usually done in private — in public restrooms before prayer, more often than during other times of the year.
And on social media, Twitter brought back the Ramadan-related hashflags. Three custom emoji appear when a user tweets the #Ramadan, #Eid, #EidMubarak or #iftar hashtags.
Clarification (June 18, 2015): The original version of this story was unclear on the timing of the Ramadan fast. The fast begins at dawn, the time that marks the beginning of the twilight before sunrise. The story has been updated. The Associated Press contributed to this report.