Finally, it behooves us to recall that the Proper chants of the Mass are linked to the liturgical seasons and times, not just in a general fashion, but quite specifically, by virtue of their content. The oldest choir-books of the Roman liturgy eloquently testify that the overwhelming majority of these chants belonged to fixed days, and these assignments remained untouched up until 1968. The same texts were written in the Missals, and if they were not sung, then the priest prayed them. In doing so, the Church clearly expressed her desire that each chant stand in a fixed position, which simply means that on this day, at this liturgical position, this is the chant, and not any other.
Exactly when and how this “properization” of the Mass chants was achieved is an altogether different question. At this point, we are not interested in this question, nor in deciding whether or not the numerous speculations are true concerning the justification of the given position of a chant and its interrelationships with other parts of the daily liturgy through historical facts or spiritual reflections. We simply accept the fact that in the minds, hearts and memories of faithful Catholics there gradually emerged, over a period of 1200 years or more, a network of associations between the experience of a particular liturgical day and the chants “proper” to that day. Such associations were truly “catholic,” in other words universal within the Latin liturgy. All felt a part of it, anyone at will referred to it: the Sundays were named after their Introits (e.g. Laetare, Gaudete, Quasimodo); people dated their private letters by referring to the same chant; composers created music not to texts, but to the Offertory or the Introit of a given day. For a Christian who lived in and with the liturgy of the praying Church, this order of chants coalesced with the full liturgy of the day, and it contributed to the high degree of constancy in the Mass Propers (as opposed to the frequent variations in the Divine Office). So it is by no means an accident that certain chant forms were excluded from this uniformity. In spite of the unchanging stability of Introits, Graduals, Offertories and Communions, the Alleluja and the Sequence presented a wide field of opportunity for the creative forces of various geographic regions (tropes, sacred polyphony).
This universality and continuity in space and time bore rich fruit, and brought great blessings. Over and above the psychological associations, such universality nurtured a feeling of stability and promoted the reverence of which a long tradition is worthy. It radiated, and thus taught, discipline; it made palpable a kind of “impersonal anonymity” which cannot be achieved simply by concealing the authors’ names. My university students were always shocked to open Dom Hesbert’s Antiphonarium Missarum Sextuplex or the 11th-century Gradual of the Roman basilica of St. Cecilia, only to find there, on the same days, the same Proper chants than the ones printed in the Liber Usualis of 1950. And without any coaching from me, their first question after the initial surprise was, “Then why should we sing different ones instead of these?”
Laszlo Dobszay, The Bugnini-Liturgy and the Reform of the Reform (2003)