Formerly obligatory feasts can be celebrated on their original days - official

But only if they are Traditional Masses. See the New Liturgical Movement for the LMS press release on the official response of the Ecclesia Dei Commission, which contradicts the suggestion of the Liturgy Office of the Bishops' Conference of England and Wales that the actual feast days have been transferred. On the contrary, when these feasts are celebrated on the Sunday - as requested by the bishops - they are celebrated as 'external solemnities', something possible and in fact quite common under the 1962 rules. (I.e., for the benefit of the faithful unable to make an important feast during the week, it can be repeated, a bit like a votive Mass, on the Sunday.) The actual day is the traditional day - as will be reflected in the breviary, by vigils, octaves and so forth.