13 Feeling themselves vindicated and defended by the Supreme Authority on earth, Christian workers received this Encyclical with special joy. So, too, did all those noble-hearted men who, long solicitous for the improvement of the condition of the workers, had up to that time encountered almost nothing but indifference from many, and even rankling suspicion, if not open hostility, from some. Rightly, therefore, have all these groups constantly held the Apostolic Encyclical from that time in such high honor that to signify their gratitude they are wont, in various places and in various ways, to commemorate it every year.
Source: Quadragesimo Anno (Vatican.va)