Here is a great article from the National Catholic Register on modesty at Mass:
Jose Guadalupe Martin Rabago, the archbishop of Leon, Mexico, has called on Catholics to dress respectfully when attending Mass. He made the request while speaking to reporters this summer after he was asked why posters were placed at the entrances of Leon parishes requesting that women not enter wearing miniskirts, sleeveless shirts or low-cut blouses. The archbishop said, “If you have any respect for [a church], dress appropriately. This is not a misogynist attitude of any sort. I am simply asking for the dignity and decorum that this place calls for, that is all.”
He added that men, too, need to be respectful in their choice of dress before entering a house of worship.
Bishops in other parts of the world have stressed the need for appropriate dress in church as well. In 2007, the Archdiocese of Manila, Philippines, issued a dress code for Mass, which included posters for parishes indicating appropriate dress. Davao, Philippines, Archbishop Fernando Capalla has made a point of reminding his parishioners of the code, stating, “Everyone should be able to dress up simply and decently [but not necessarily] elegantly for Mass. ... Beach wear and working-out outfits are not for solemn and sacred celebrations like the Mass.”
More than just making a request, the Vatican has insisted that tourists visiting St. Peter’s Basilica in Rome adhere to a strict dress code. Pictorial signs explaining the dress code are on display at entrances; men and women in shorts or with bare shoulders (e.g. wearing tank tops) are routinely turned away by the Swiss Guards.
The primary motivation for such policies, say those in leadership roles in the Church, is to promote the virtue of modesty. The Catechism of the Catholic Church offers several paragraphs on the topic of modesty (2521-2524), saying in part: “Modesty is decency. It inspires one’s choice of clothing. … Modesty inspires a way of life that makes it possible to resist the allurements of fashion and the pressures of prevailing ideologies.”
Many of the popes have spoken out about the importance of modesty, including Pope John Paul II in his book Love and Responsibility: “Man, alas, is not such a perfect being that the sight of the body of another person … can arouse in him merely a disinterested liking which develops into an innocent affection.”
Chastity/modesty speaker and Catholic Answers’ apologist Leah Darrow, once a successful model and contestant on America’s Next Top Model, added, “Although the Church has no dogmatic teaching on modest dress, it exhorts us to use common sense. If we are invited to a gala event, we don’t dress like we were invited to a beach party. Christ himself has invited us to partake in the sacred banquet of the Eucharist. Should not our attire and our reverent actions reflect where we are and who we are in the presence of? Reflect upon the Crucifixion; the invitation has been sent. Let us respond promptly and appropriately.”
It was a desire to teach her teenage daughter the virtue of modesty that led Martha Fernandez to request a dress code be adopted at her parish, the National Shrine of Our Lady of Guadalupe in Sacramento, Calif.