Yes, as odd as that may sound, in a few months we shall note the FIFTIETH ANNIVERSARY of "Subsidiarity".
Because on April 11, 1963, Pope John XXIII released his encyclical Pacem in Terris, in which the term "subsidiarity" was used (in its modern meaning) for the first time. A text copy can be found here; see section 140 for the term. But the idea is older, and so is the original term - and if you want further details, please check out the book Subsidiarity, available here.
Please note: while we shall note that day in some fashion, our own special celebration, however, will occur on March 2, which is the "birthday" of Subsidiarity, the day in 2000 when our system went live. More on that at the proper time.