What was the reasoning behind the suppressing of many Vigils and Octaves under Pope Pius XII?
It is indeed true that Pius XII abolished several of the Churchs Vigils and Octaves that had been observed for many centuries before. The reason given by the Sacred Congregation of Rites in its decree of March 23, 1955, to be effective as of January 1, 1956, was the simplification of the rubrics of the Roman Breviary and Missal. It is certainly true that the rubrics prior to that time were quite complicated, especially when it became a question of overlapping octaves, and that it was a legitimate aspiration of the liturgical movement to simplify these rubrics in such a way that the ordinary faithful could follow, understand and participate. Furthermore, periodic elimination of added feasts, octaves and other liturgical days are not unusual in the history of the Church. Consequently, it is certainly successive to call this elimination of Octaves and Vigils a “modernist innovation” In fact the three most important Octaves are the ones that were retained in 1955, namely those of Christmas