What is Ramadan -- and why do Muslims fast?

Muslims around the world are celebrating the start of Ramadan, the 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.

There was an estimated 3.45 million Muslims in America making up about 1.1 percent of the population in 2017, according to the Pew Research Center.

Ramadan ends with Eid al-Fitr, the feast of fast-breaking. It's a generally 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.

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.

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 nursing mothers, travelers, the elderly and the sick are generally 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.

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 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 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.