Muslims offer the first Friday prayers of the holy month of Ramadan inside a mosque in Allahabad, India. Reuters

Worshipers flooded mosques across the world this week as the Muslim holy month of Ramadan got underway. Ramadan 2015 runs through July 17. For its duration, practicing Muslims will fast during daylight hours and attend extra prayers, often traveling great distances to holy sites.

Ramadan happens every year during the ninth month of the Islamic calendar, Hijiri. It's based on the lunar cycle, so the observance officially begins when followers see the crescent moon -- hilal -- emerge in the sky. The hilal was spotted in many places on Wednesday, meaning Ramadan began Thursday in places like Qatar, Jordan and the United Arab Emirates. India and Pakistan saw the hilal Thursday, deciding to start Ramadan on Friday, the News Teller reported.

In Jerusalem, home of Al-Aqsa Mosque, Palestinian and Israeli police collaborated to control the crowds. They expected thousands of people from Ramallah, Bethlehem and Hebron, all of which have large Muslim populations, Israeli police spokesman Micky Rosenfeld told the Jersualem Post. Worshipers also flocked to the shrine at the nearby Dome of the Rock, the site where legend says the Prophet Muhammad ascended to heaven.

A Palestinian woman prays in front of the Dome of the Rock on the first Friday of the holy month of Ramadan in Jerusalem's Old City on Friday. Reuters

"We're excited as always to make the journey," a 64-year-old identified as Ahmed told the Daily Star. "It took a while to get through the checkpoint ... but it was worth it."

More than 1.5 million people visited Saudi Arabia's Mecca and Medina, known as the two holiest sites in Islam, on Thursday, Arab News reported. Authorities there hired 12,000 people to keep the grounds clean and the crowds orderly. Mecca Gov. Khalid Al Faisal Al Saud organized public transportation for the worshipers. The 1,600 buses could make as many as 5 million trips during Ramadan, Al Arabiya reported. The King Abdulaziz International Airport was prepared for 20,000 flights full of pilgrims.

Men and children attend an evening mass prayer session to mark the holy fasting month of Ramadan, along a road in Karachi, Pakistan, on Thursday. Reuters

Nightly during Ramadan, mosques host special prayers called taraweeh, a word with Arabic roots meaning "rest" or "relaxation." Taraweeh often takes more than an hour, as leaders read parts of the Quran out loud. Worshipers cycle through many prayer positions during taraweeh -- they stand, sit, bow and prostrate.

Men lined up for Friday prayers on the first Friday of Ramadan. Reuters

Most Muslims also fast during Ramadan, with the exception of children, the sick, the elderly, pregnant women and travelers. Fasting gives people a chance to reflect, practice discipline and cleanse themselves of impurities. Muslims often give to charities during Ramadan, as well, according to the Council on American-Islamic Relations.

Muslims attend Friday prayers during the second day of Ramadan in the courtyard of a housing estate next to the small BBC community center and mosque in east London on Friday. Reuters