The deliberation of the synod about transferring to Sunday feasts distributed through the year, and rightly so, because it is convinced that the bishop has power over ecclesiastical discipline in relation to purely spiritual matters, and therefore of abrogating the precept of hearing Mass on those days, on which according to the early law of the Church, even then that precept flourished.That is very interesting. I did not know that the Jansenists were in favour of transferring Holydays to the nearest Sunday. So many liturgical changes of recent decades were enthusiastically promoted by the Jansenists.
Pius VI Auctorem Fidei 1794 n.74
To avoid confusion: the condemnation of this proposition is not a matter of infallibility in dogmatic matters but a disciplinary provision, particularly concerning the authority of Bishops. The Holy See has in recent decades subsequently given the necessary permission for Bishops' Conferences to transfer Holydays to Sunday.