15 May 1891. Vincenzo Gioacchino Raffaele Luigi Pecci (20 Feb 1878-20July 1903), more popularly known as Leo XIII, publishes his social encyclical, Rerum Novarum ("of new things"). According to Richard McBrien, the document "remains a benchmark of Catholic social teaching." 
- McBrien, Richard P. 1997. Lives of the Popes : The Pontiffs from St. Peter to John Paul II. CA:HarperSanFrancisco.