Please look back one year, to my posting of the chant, the text and the Scriptural sources of to-night's Major Antiphon, O Radix Jesse...
I suspect, by the way, that these are called the Antiphonæ majores, or "greater anthems", both because of their relative length - rather longer than the ferial antiphons for the Magnificat, though they are by no means as lengthy nor as musically elaborate as some (such as the Hodie antiphons for some of the great feasts of the Church's Year), and because of their pregnancy with meaning, singing of the high office and titles of the Messias.
In cathedrals, collegiate churches, monasteries and priories of old - and, albeit in too few places, even to-day - each evening leading up to Christmas was marked with the singing of these several anthems, always doubled (sung in full before and after the Gospel Canticle), if not tripled, and conjoined with the Magnificat on a solemn tone, while the celebrant fumed the altar. This was a dramatic upping of ceremonial and chant for what are otherwise ferial days of Advent!
Furthermore, it was appointed by custom that each night, as Christmas drew nearer, the O Antiphon was to be intoned by an official of greater and greater rank - in an abbey, ending with the Abbot himself, after the Prior, the Deans, and the other monastics in due order.
Christians having a true hierarchical sensibility - far from delusions of domineering power - will appreciate this; for even, before and above all, the Blessed Trinity consists of a due order of Persons, without any inequality. "Let all things be done decently and in order", writes the Apostle (I Cor. xiv, 40).