It's traditionally given on the 29th of June, the feast day of Saints Peter and Paul, apostles.
Archbishop Chaput will be among four archbishops from the United States and 45 from around the world to receive the pallium.
The good folks at the U.S. Conference of Catholic Bishops put together this primer on the pallium and its recipients this year.
One man with over 25 years of experience wearing one, Cardinal Roger Mahony, the retired archbishop of Los Angeles, explains:
It is a circular band about 2" wide, worn about the neck and having two pendants--one hanging down in front and one behind. It is worn over the chasuble at Mass. Every February two lambs are blessed each year and their white wool is used to make the Pallium. The wool is presented to the Pope, and Sisters then make the Pallium for the new Archbishops. ... The new Palliums are solemnly blessed on the eve of the feast of Saints Peter and Paul, and are kept in a special silver-gilt container in front of the Main Altar in St. Peter's Basilica.
What's a metropolitan archbishop? In the Philadelphia region that Archbishop Chaput leads, for example, the Archdiocese is considered the metroplitan "see" of all the other eight dioceses in Pennsylvania. As such, the Pope wishes to show his special bond with the arhcbishop on behalf of all the Catholic faithful not only in that archdiocese, but all the dioceses in that metropolitan region.
The giving of the pallium and presence of the archbishop with the Holy Father is a good occasion for Catholics to go to Rome on a spiritual pilgrimage to some of the Church's holiest, most ancient places.
Many faithful from Philadelphia and Denver, where Archbishop Chaput led that local church for 14 years, are in Rome this week for the liturgy and, probably, not a bad meal for the duration.