2 Part of this large body of social teaching is the distinguished Encyclical Populorum Progressio , which my esteemed predecessor Paul VI published on March 26, 1967. The enduring relevance of this Encyclical is easily recognized if we note the series of commemorations which took place during 1987 in various forms and in many parts of the ecclesiastical and civil world. For this same purpose, the Pontifical Commission Iustitia et Pax sent a circular letter to the Synods of the Oriental Catholic Churches and to the Episcopal Conferences, asking for ideas and suggestions on the best way to celebrate the Encyclical's anniversary, to enrich its teachings and, if need be, to update them. At the time of the twentieth anniversary, the same Commission organized a solemn commemoration in which I myself took part and gave the concluding address. And now, also taking into account the replies to the above-mentioned circular letter, I consider it appropriate, at the close of the year 1987, to devote an Encyclical to the theme of Populorum Progressio.
Source: Sollicitudo Rei Socialis (Vatican.va)