The point is, going to Mass on Sundays and Holy Days are not Divine Law. The Church made it obligatory that we assist at Mass that day
There is no contradiction between Divine Law and Church Law. The Divine Law commands we offer sacrifice to God. In the New Testament, such sacrifice is Holy Mass.
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The old law required an annual sacrifice for sin; similar to our annual requirement to go to confession once a year. But could the Church get rid of this annual confession rule? I would say ‘no’, because it has its roots in the Old Testament, which came from God (ie Divine Law).
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In the same way, the weekly requirement for Sunday Mass is a Church Law, yes, but does that mean it ORIGINATED with St Peter? In other words, is the Sunday Mass requirement a creation of the Church, which could be done away with entirely? No, it cannot. The requirement for Sunday Mass would have to be considered part of Tradition, ie an order from Christ to the Apostles, so it is Divine Law, made know through the Church.
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Holy Days of obligation are strictly Church laws and almost all (except Christmas, which feast has been celebrated since Apostolic Times) could be, in theory, removed from the calendar.