The World Meeting of Families is scheduled to officially be brought to a close Sunday when Pope Francis visits Philadelphia and celebrates Mass with an estimated hundreds of thousands of devotees in attendance. Aimed at bringing together prayer and faith lectures, the conference is described by organizers as the world's largest gathering of Catholic families, according to WPVI-TV in Philadelphia.

The event is held every three years and is sponsored by the Holy See’s Pontifical Council for the Family. It was first conceived in 1992 by Pope John Paul II in an effort to strengthen "the sacred bonds of the family unit across the globe," according to the World Meeting of Families website. In 1994, the first event took place in Rome. Ever since, families have been invited to "share their thoughts, dialogue and prayers, working together to grow as individuals and family units," the website reads.

The 2015 conference is expected to be the largest in history, with more than 18,000 people signed up. That figure is double the total of the last World Meeting of Families held in 2012 in Milan. More than 100 nations will be represented at the 2015 event, with the United States, Canada, Mexico, Pakistan and Vietnam leading the pack in registrants, according to WPVI.

The conference, which in 2015 has the theme of "Love Is Our Mission: The Family Fully Alive," began Tuesday in Philadelphia to much excitement. "It has begun -- the start of the most historic event in modern Philadelphia history ... I want to welcome the joyous pilgrims here to our great city," Philadelphia Mayor Michael Nutter said to tens of thousands who had gathered at the Pennsylvania Convention Center to kick off the event, according to the Philadelphia Inquirer

Throughout the week, the World Meeting of Families is expected to hold a number of workshops, talks and conferences for adults and children that will feature notable speakers and focus on social and religious issues. Some of the topics for sessions include interfaith marriage, divorce, "hook-up culture" and sexuality, according to WPVI.