Reform of the rubrics and liturgical calendar

The rubrics and calendar of the Mass and the Divine Office were reformed by the constitution cuм hac nostra aetate (March 23, 1955). The reform to the calendar, the most dramatic before its complete overhaul in 1969, consisted mainly in the abolition of various octaves and vigils. An octave is the week-long prolongation of a great feast, either by the celebration of a proper Mass all through the Octave or by the addition of an additional Collect when the Mass of another feast is celebrated. Of the 18 octaves existing in the Roman calendar, all but three (Easter, Pentecost, Christmas) were purged in the reform, including the octaves of the Epiphany, Corpus Christi, the Ascension and the Immaculate Conception. A vigil is a day of liturgical preparation preceding a great feast. The reform of 1955 eliminated roughly half the vigils in the Roman calendar, including the vigil of the Epiphany and the vigils of the Apostles.
The ancient custom of beginning a feast with first Vespers on the eve of the feast was abolished, with certain exceptions. Following the reform, most feasts have only one set of Vespers (what formerly was known as second Vespers), celebrated on the afternoon of the feast itself. The purpose of this derogation of the ancient custom was to simplify the process by which a feast had to be commemorated when the second Vespers of one feast coincided with the first Vespers of the subsequent feast. The reform also abolished the custom whereby Vespers was to be recited before noon during Lent. This custom was a survival of the ancient custom whereby the Lenten fast could only be broken after Vespers; the Church had long since permitted this meal to be taken at mid-day and had thus also anticipated the office of Vespers during Lent.
Proper Last Gospels were also eliminated in the reform, with the exception of the third Mass of Christmas (when the Gospel of the Mass is taken from John 1) and at Low Masses on Palm Sunday. A "Proper Last Gospel" occurs when a commemoration is made at Mass of another feast (or feria or vigil or Sunday) of a high rank, whose Gospel is read at the end of Mass in place of the habitual Last Gospel (John 1: In principio). Prior to the reform of Pius XII, a proper Last Gospel was always said when a feast was celebrated instead of a feria of Lent, or a vigil, or a Sunday.
The manner of ranking feasts was also changed slightly. The reform of 1955 suppressed the rank known as the semi-double, leaving only doubles and simples. All semi-double feasts became simples, and all semi-double Sundays became doubles. (In 1960 John XXIII completely replaced the traditional manner of ranking feasts by abolishing the double, with its various grades, and the simple, and classifying feasts instead as first, second, third, or fourth class.)
Finally, the supplementary prayers formerly recited in connection with the breviary were also suppressed. Thus, for example, the various seasonal Marian antiphons formerly recited at the end of the liturgical hours were retained only after Compline.
In his book The Simplification of the Rubrics, explaining the changes, Monsignor Annibale Bugnini commented, "The present decree has a contingent character. It is essentially a bridge between the old and the new, and if you will, an arrow indicating the direction taken by the current restoration."