The simplest explanation of an octave is that it is the extension of a particularly important feast day to span eight days -- hence, "octave." Presently we are sitting within an octave in point of fact: the Octave of Easter; thus, for eight days from Easter Sunday, we liturgically we sit within an extension of that great feast, the day of the Resurrection. From a practical point of view then, this ancient practice allows us to sit within, relish and meditate upon the mysteries associated with this great feast for a span of eight days -- rather than simply observing the feast day and then move on to the next thing in the liturgical calendar.
Feasts that had octaves included, most anciently, Easter and Pentecost, and then other commemorations of salvation history, such as Christmas, the Ascension and Epiphany. Others commemorated patronal saints important to the local or universal church, such as Ss. Peter and Paul, All Saints and so on.
Why eight days? In Jewish practice, the number seven had particular importance and in Christian practice, developing from this, the "eighth day" was associated with Sunday, the day of the Resurrection, and by extension, baptism as well.
But if that is the function of an octave, what of their history? Octaves began to first appear around the time of the Emperor Constantine when eight day long festivities were observed for the erection of certain important basilicas, but these were singular events, not annual ones. The first formalized liturgical octaves as we know them appear also around the time of Constantine, in the 4th century, in relation to Easter, Pentecost and Epiphany. Following this, Christmas was granted an octave and, going back to the Constantinian roots of the practice, the Roman church also instituted an octave for the feast of the dedication of a church.
From about the 7th century, certain important saints feasts days began to have lesser octaves observed, most anciently the feast of Ss. Peter and Paul, St. Lawrence and St. Agnes -- feasts of particular devotion and importance to the Roman church.
As intimated earlier, this aspect of the local church also entered into the octave equation as a particular diocese or even monastery might observe octaves for their own patronal saints. This was common in the middle ages.
This period of the growth of octaves from the 4th through 15th centuries was then met by a period of culling, beginning with the Tridentine reforms under St. Pius V. Here, a new classification of the type of octave (privileged, common and simple octaves) came into being. The purpose of this classification system was to distinguish between those octaves that were the most focused and uninterrupted liturgically speaking (i.e. which ones were the literal extension of a feast day through eight days) and those which were lesser, permitting other high ranking feasts to 'interrupt' the octave. By the mid 20th century, under the liturgical reforms of Pius XII, none but the three privileged octaves of Easter, Christmas and Pentecost were kept; then, under Paul VI's reforms, Pentecost, one of the most ancient octaves of all, even more ancient than that of Christmas, was further eliminated. So it is today that in the calendar of the usus antiquior, three octaves are observed, and in the modern rite, only two.
It is, of course, precisely because of the venerable antiquity of the octave of Pentecost that makes its exclusion in the Pauline liturgical calendar, without question, the most controversial exclusion of all. The calendar of the usus antiquior is, fortunately, preserved from this specific controversy, but even here, questions remain about the propriety of removing the ability of a local church to observe octaves of particular importance locally, or of the Roman church's observance of Ss. Peter and Paul, never mind octaves of Christological importance like those of Epiphany or the Ascension. But I digress. Our intent here today is not to debate those specifics, but to perhaps provide a bit of the background of the practice of octaves generally -- and indeed, perhaps this will also help to inform our considerations of this prickly question.
