27 In the first place it must be noted that our predecessors have always shown the greatest zeal in protecting and preserving the lawful rites of the Ruthenians. For when the bishops, through the bishops of Vladimir and Lutsk who were sent to Rome on this business, asked "that His Holiness, for himself and his successors, who were never to make any change in this matter, would deign to preserve and confirm to the Eastern Church its administration of the sacraments, its rites and ceremonies, involate and entire, as in use at the time of the union." Clement VIII graciously acceded to their petition, and prescribed that no alteration was to be made in these matters. Not even the use of the new Gregorian calendar was afterwards imposed on them, although it seemed at first that the Ruthenians ought to employ it while retaining the liturgical calendar of the Eastern rite; so that even now the Julian calendar can remain in force among them.
Source: Orientales Omnes Ecclesias (Vatican.va)