Islam's Holiest Month of Ramadan Begins, Here's What It's All About

— -- Grumbling stomachs, headaches, weakness and exhaustion. These are some of the physical symptoms many Muslims across the world will experience as they embark on a spiritual journey to cleanse and detox their souls.

Today marks the first day of Islam's holiest month: Ramadan. For the next 30 days, most of the world’s 1.6 billion Muslims will abstain from food and drink in a month meant for contemplation, self-discipline and remembrance of God.

What Is Ramadan?

Ramadan is the ninth month in the Islamic lunar calendar when Muslims, followers of Islam, adhere to a strict fast observed from sunrise to sunset. For example, a Muslim in the Eastern Time Zone will be fasting for roughly 15 hours a day this year.

It’s considered a sacred month in Islam because it’s when Muslims believe the first verses of the Quran, Islam’s holy book, were revealed to the Prophet Muhammad more than 1,400 years ago.

Ramadan is also considered to be a time of intense prayer, good deeds and charity.

Why Do Muslims Fast During the Month of Ramadan?

Fasting is viewed as a time to exercise self-control, and as a cleanse for the mind, body and spirit. Many Muslims liken the fasting to a spiritual detox, a way to bring themselves closer to God. The fasting is also intended to act as a reminder of the suffering of those less fortunate, who often don't have access to food and water.

Fasting is one of the five pillars of Islam. The other pillars are to pray five times a day, the declaration and testimony of faith, supporting the needy and the pilgrimage to Mecca, Islam's holiest city, at least once in a Muslim's lifetime.

Fasting includes not eating and drinking anything during daylight hours: no water, not even medicine. The faithful also abstain from smoking and sex.

All Muslims are expected to follow these guidelines but there are exceptions for the elderly, the sick, pregnant women and young children.

During the month of Ramadan, it’s common for Muslims to gather with friends and families for suhoor, the first meal of the day eaten just before the sun rises. They’ll also gather at the end of the day for iftaar, the breaking of the fast just after the sun sets.

In July, Muslims will celebrate Eid al-Fitr, a religious celebration marking the end of Ramadan.

Obama Offers His Best Wishes

President Obama issued a statement Sunday, offering his best wishes to Muslims in the United States and around the world.

"For many, this month is an opportunity to focus on reflection and spiritual growth, forgiveness, patience and resilience, compassion for those less fortunate, and unity across communities. Each lesson is profound on its own, and taken together forms a harmonious whole," he said in a written statement.

He also took the opportunity to make a thinly veiled swipe at Republican presidential nominee Donald Trump, who has suggested a temporary ban to prevent Muslims from entering the country.

"I stand firmly with Muslim American communities in rejection of the voices that seek to divide us or limit our religious freedoms or civil rights," Obama said. "I stand committed to safeguarding the civil rights of all Americans no matter their religion or appearance. I stand in celebration of our common humanity and dedication to peace and justice for all."

Ramadan Well-Wishes a la Twitter

Some politicians have offered their well-wishes to Muslims, including Hillary Clinton.

Rep. Keith Ellison, D-Minn., also tweeted out his support. Ellison is the first Muslim to be elected to the U.S. Congress.

London Mayor Sadiq Khan also posted on Twitter, calling for a "peaceful" month of Ramadan.

Khan, a former human rights lawyer and a son of a bus driver from Pakistan, is the first Muslim to lead the British capital. Writing Sunday in The Guardian newspaper, Khan acknowledged that fasting for Ramadan won't be easy and "it's the coffee I'll miss the most."