What is the meaning of "Double Feast?" Does this mean that we need to chant a Gloria and Credo?
"Double Feast" refers to an older system of ranking the levels of feasts, which was superseded by the simplified system of ranking implemented by Pope John XXIII in the motu proprio Rubricarum instructum in 1960.
Under this newer system of ranking, the feast of St. Alphonsus Ligouri is a feast of the third class (unless yours is a Church dedicated to St. Alphonsus or a Redemptorist Church).
A third class feast has a gloria, but no credo. The festal, rather than the ferial tones, are used for the preface and other prayers.
(There could be a while discussion about what "double feast" meant in terms of the history of the liturgy, but I'll omit that here to focus on the practical question.)
Before the liturgical reforms of the mid-twentieth century, the Credo was also sung at feasts of Doctors of the Church (as St. Alphonsus is), even when they ranked below the level where they would usually require a creed, but this practice was eliminated.
Looking for the psalm verses to be sung with the communion Sacerdos magnus, it struck me that the chants in the 1974 Graduale Romanum are completely different from the ones prescribed by the Ordo cantus missae. Hence, there are no suggestions for an accompanying psalm to be found in the Graduale for this antiphon.
But as the antiphon is taken from Sirach 50:1.9, verses from psalm 96 (95) might be fit.
Sacredos magnus - is it known what is the prescribed psalm for this communion antiphon?
The antiphon is taken from Ecclesiasticus 50:1&9, so following the rubric that "If the Antiphon is not taken from a Psalm, some Psalm suitable to the feast and to this part of the Mass may be chosen," you get to pick if you want to sing verses.
One possibilities might be Psalm 132, Ecce quam bonum, used for a votive of St. Alphonsus during Easter as the first Alleluia.
Just to clarify, I think the use of that book is permitted, but it's selections are just those of the editors, right? They're not binding, it seems? The "some Psalm suitable" rubric would still apply.
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