One of the world’s most historic and religious mass gatherings begins Saturday.

Millions of Muslims made their Hajj pilgrimage this week to the Islamic holy city of Mecca, Al Jazeera reported. The pilgrimage is an important spiritual journey meant to cleanse faithful Muslims of sin and to bring them closer to God. The annual journey is deemed a “central pillar of the Islamic faith.” The Islam religion has five pillars -- or basic acts of faith -- and hajj is the fifth.

RTX25ABC Muslims touch and pray at the door of the Kaaba and touch and kiss the al-Hajr al-Aswad "Black Stone" during their Umrah Mawlid al-Nabawi "Birthday of Prophet Mohammad" in the holy city of Mecca, Saudi Arabia Jan. 22, 2016. Photo: REUTERS/Amr Abdallah Dalsh

This year, Hajj ends on Wednesday in some Muslim nations. The holiday follows the Islamic calendar and takes place during the final month on the calendar, which shifts every year, about 11 days earlier than the Gregorian calendar.

RTS2C09 A Hajj returnee from Saudi Arabia, Kogi State, holds a Koran Sept. 29, 2015. Photo: REUTERS/Afolabi Sotunde

Being the fifth pillar of the Islamic faith, hajj is obligatory for every Muslim, and it is recommended that the journey is made at least once in every Muslim’s lifetime. The journey allows practicing Muslims to renew their devotion to God.

RTX1S7WX A view of the camp city at Mina, near the holy city of Mecca Sept. 24, 2015. Photo: REUTERS/Ahmad Masood

“Hajjis” arrive at Mecca where they pray and participate in a variety of rituals. One such ritual is the circling of the holy Kaab, a large, cube structure that is considered the house of God.

RTX25AB8 Muslims touch and pray at the door of the Kaaba and touch and kiss the al-Hajr al-Aswad "Black Stone" during their Umrah Mawlid al-Nabawi "Birthday of Prophet Mohammad" in the holy city of Mecca, Saudi Arabia Jan. 18, 2016. A similar ritual takes place over hajj. Photo: REUTERS/Amr Abdallah Dalsh

Men and women are permitted to sit together at the Grand Mosque during hajj.

Men are traditionally required to wear two pieces or white sheets, while women are to wear a hijab that covers both neck and hair.

RTX1S650 Muslim pilgrims arrive to cast stones at pillars symbolizing Satan during the annual hajj pilgrimage in Mina on the first day of Eid al-Adha, near the holy city of Mecca Sept. 24, 2015. Photo: REUTERS/Ahmad Masood

The Hajj always coincides with Eid al-Adha, another major Muslim holiday. The holiday begins on the 10th day of Dhul Hijjah and, traditionally, a large animal is slaughtered and portions of it are distributed to the poor, reported Pew Research. The sacrifice is meant to commemorate the story of Abraham’s sacrifice to God in the Quran.

RTX240VF Children watch men slaughtering sheep on the first day of Eid al-Adha in Aleppo's Tariq al-Bab neighborhood, Syria Sept. 24, 2015. Muslims around the world celebrate Eid al-Adha to mark the end of the Hajj pilgrimage by slaughtering sheep, goats, camels and cows to commemorate Prophet Abraham's willingness to sacrifice his son, Ismail, on God's command. Photo: REUTERS/Abdalrhman Ismail