http://www.slate.com/articles/news_and_politics/jurisprudence/2015/10/supreme_court_justices_at_red_mass_and_pope_francis_address.html

For anyone who was counting along at home when Pope Francis gave the first address by a leader of the Catholic Church to a joint meeting of the U.S. Congress last month, only four Supreme Court justices showed up (Chief Justice John Roberts and Justices Anthony Kennedy, Ruth Bader Ginsburg, and Sonia Sotomayor). Justices Antonin Scalia, Clarence Thomas, and Samuel Alito, all Catholics, did not attend, which generated some buzz on the Internet. Their absence was not really all that surprising given their tendency to skip joint sessions in general. (Scalia has avoided them for almost 20 years, dismissing them as a “childish spectacle” to which he doesn’t want to “lend dignity.”)