New Liturgical Movement, Gregory Dipippo: : In the Extraordinary Form, the Immaculate Conception takes precedence over the Second Sunday of Advent, which is reduced to a commemoration at the Mass of the former. This is a special exception made solely for this feast, which is part of the rubrics of the 1962 Missal (Rubr. Gen. III, 15), and which is NOT superceded by any subsequent canonical or rubrical legislation, including the rubrics of the Ordinary Form. As stated in paragraph 28 of the Instruction Universae Ecclesiae, which was issued in 2011 to clarify certain issues related to the motu proprio Summorum Pontificum, “Furthermore, by virtue of its character of special law, within its own area, the Motu Proprio Summorum Pontificum derogates from those provisions of law, connected with the sacred Rites, promulgated from 1962 onwards and incompatible with the rubrics of the liturgical books in effect in 1962.” Read more on this here.
