the universal magesterium of the Church in establishing the Roman Calendar establishes solemnities, feasts, memorials of saints etc., and the “master list” of those solemnities which carry the obligation to assist at Mass. the Bishops Conference of each country decides which of those days are obligatory in their country, and the bishop of each diocese can further make changes in his own diocese, for instance, ruling that if a certain holy day falls on Saturday or Monday the obligation is removed.
