9 We decree, therefore, that on October 12, or on the following Sunday, if the Ordinary should prefer it, in all the Cathedral churches and convent chapels throughout Spain, Italy, and the two Americas, after the office of the day there shall be celebrated a Solemn Mass of the Most Holy Trinity. Moreover, besides the abovementioned countries, We feel assured that the other nations, prompted to it by the counsel of their bishops will likewise join in the celebration, since it is fitting that an event from which all have derived benefit should be piously and gratefully commemorated by all.
Source: Quarto Abeunte Saeculo (Vatican.va)