The Introit for Pentecost Ember Friday: Repleatur os Meum ("Let my mouth be filled with your praise")

Here's a recording of this beautiful introit, sung by the "Choralschola of the Niederaltaicher Scholaren":



The text is taken from various verses of Psalm 71:

Repleatur os meum laude tua, Domine. Alleluia.
Ut possim cantare. Alleluia.
Gaudebunt labia mea dum cantavero tibi. Alleluia.
In te, Domine, speravi,
non confundar in aeternum:
in iustitia tua libera me (et eripe me).

Let my mouth be filled with thy praise, O Lord. Hallelujah.
That I may sing. Hallelujah.
My lips shall rejoice when I sing to you. Hallelujah.
In you, O Lord, do I take refuge;
let me never be confounded:
In your righteousness deliver me and rescue me.

And what is "Pentecost Ember Friday," you ask?  Here's an explanation from the Catholic Encyclopeida of 1913; my bolding below:

Ember days (corruption from Lat. Quatuor Tempora, four times) are the days at the beginning of the seasons ordered by the Church as days of fast and abstinence. They were definitely arranged and prescribed for the entire Church by Pope Gregory VII (1073-1085) for the Wednesday, Friday, and Saturday after 13 December (S. Lucia), after Ash Wednesday, after Whitsunday, and after 14 September (Exaltation of the Cross). The purpose of their introduction, besides the general one intended by all prayer and fasting, was to thank God for the gifts of nature, to teach men to make use of them in moderation, and to assist the needy. The immediate occasion was the practice of the heathens of Rome. The Romans were originally given to agriculture, and their native gods belonged to the same class. At the beginning of the time for seeding and harvesting religious ceremonies were performed to implore the help of their deities: in June for a bountiful harvest, in September for a rich vintage, and in December for the seeding; hence their feriae sementivae, feriae messis, and feri vindimiales. The Church, when converting heathen nations, has always tried to sanctify any practices which could be utilized for a good purpose. At first the Church in Rome had fasts in June, September, and December; the exact days were not fixed but were announced by the priests. The "Liber Pontificalis" ascribes to Pope Callistus (217-222) a law ordering the fast, but probably it is older. Leo the Great (440-461) considers it an Apostolic institution. When the fourth season was added cannot be ascertained, but Gelasius (492-496) speaks of all four. This pope also permitted the conferring of priesthood and deaconship on the Saturdays of ember week--these were formerly given only at Easter. Before Gelasius the ember days were known only in Rome, but after his time their observance spread. They were brought into England by St. Augustine; into Gaul and Germany by the Carlovingians. Spain adopted them with the Roman Liturgy in the eleventh century. They were introduced by St. Charles Borromeo into Milan. The Eastern Church does not know them. The present Roman Missal, in the formulary for the Ember days, retains in part the old practice of lessons from Scripture in addition to the ordinary two: for the Wednesdays three, for the Saturdays six, and seven for the Saturday in December. Some of these lessons contain promises of a bountiful harvest for those that serve God.

Here's some of the entry for "Ember Days" from Wikipedia; unfortunately there's no mention of the very important purpose described in bold in the entry above:

In the liturgical calendar of the Western Christian churches, Ember days are four separate sets of three days within the same week — specifically, the Wednesday, Friday, and Saturday — roughly equidistant in the circuit of the year, that were formerly set aside for fasting and prayer. These days set apart for special prayer and fasting were considered especially suitable for the ordination of clergy. The Ember Days are known in Latin as the quattuor anni tempora (the "four seasons of the year"), or formerly as the jejunia quattuor temporum ("fasts of the four seasons").

The four quarterly periods during which the ember days fall are called the embertides.

Ember Weeks

The Ember Weeks—the weeks in which the Ember Days occur—are the weeks:

• between the third and fourth Sundays of Advent (although the Common Worship lectionary of the Church of England places them in the week following the second Sunday in Advent);
• between the first and second Sundays of Lent;
• between Pentecost and Trinity Sunday; and
• the week beginning on the Sunday after Holy Cross Day (September 14), the liturgical Third Week of September.

Origins

The origins of the observance are open to considerable debate. Some hold that the concept of the observance predates the Christian era, and that since Ember days have never been observed in the Eastern Churches, any pagan origins must lie in the west.^{[citation needed]} Some point to specific Celtic origins, linked to the Celtic custom of observing various festivals at three-month intervals: Imbolc, Beltane, Lughnasadh and Samhain. In any event, the ancient Christian church often sought to co-opt pagan feasts and reorient them to different purposes, and that seems to have been applicable in this instance^{[citation needed]}.

In pagan Rome offerings were made to various gods and goddesses of agriculture in the hope that the deities would provide a bountiful harvest (the feriae messis in July), a rich vintage (the feriae vindimiales in September), or a productive seeding (the feriae sementivae in December). At first the Church in Rome had fasts in June, September, and December. The Liber Pontificalis ascribes to Pope Callixtus I (217-222) a law regulating the fast, although Leo the Great (440-461) considers it an Apostolic institution. When the fourth season was added cannot be ascertained, but Pope Gelasius I (492-496) speaks of all four.

The earliest mention of four seasonal fasts is known from the writings of Philastrius, bishop of Brescia (died ca 387) (De haeres. 119). He also connects them with the great Christian festivals.

The Christian observation of this seasonal observance of the Ember days had its origin as an ecclesiastical ordinance in Rome and spread from there to the rest of the Western Church. They were known as the jejunium vernum, aestivum, autumnale and hiemale, so that to quote Pope Leo's words (A.D. 440 - 461) the law of abstinence might apply to every season of the year. In Leo's time, Wednesday, Friday and Saturday were already days of special observance. In order to tie them to the fasts preparatory to the three great festivals of Christmas, Easter and Pentecost, a fourth needed to be added "for the sake of symmetry" as the Encyclopædia Britannica 1911 has it.

From Rome the Ember days gradually spread unevenly through the whole of Western Christendom. In Gaul they do not seem to have been generally recognized much before the 8th century.

Their observation in Britain, however, was embraced earlier than in Gaul or Spain, interestingly, and Christian sources connect the Ember Days observations with Augustine of Canterbury, AD. 597, said to be acting under the direct authority of Pope Gregory the Great. The precise dates appears to have varied considerably however, and in some cases, quite significantly, the Ember Weeks lost their connection with the Christian festivals altogether. Spain adopted them with the Roman rite in the eleventh century. Charles Borromeo introduced them into Milan in the sixteenth century.

In the Eastern Orthodox Church ember days have never been observed.^[1]

Timing

The Ordo Romanus fixed the spring fast in the first week of March (then the first month), thus loosely associated with the first Sunday in Lent; the summer fast in the second week of June, after Whitsunday; the autumnal fast in the third week of September following the Exaltation of the Cross, September 14; and the winter fast in the complete week next before Christmas Eve, following St. Lucy's Day (Dec. 13).

Other regulations prevailed in different countries, until the inconveniences arising from the want of uniformity led to the rule now observed being laid down under Pope Urban II as the law of the church, at the Council of Piacenza and the Council of Clermont, 1095.
These dates are given in the following mnemonic:

Dant Crux, Lucia, Cineres, Charismata Dia

Ut sit in angariâ quarta sequens feria

Or in an old English rhyme

"Fasting days and Emberings be

Lent, Whitsun, Holyrood, and Lucie."

Another way to remember it, in the form in which I've heard it recently:  "Lenty, Penty, Crucy, Lucy"!

More from the article:

The ember days began on the Wednesday immediately following those days.

^.....

They may appear in some calendars as "days of prayer for peace".^[4]

....

Etymology

The English name for these days, "Ember", derives from the Anglo-Saxon ymbren, a circuit or revolution (from ymb, around, and ryne, a course, running), clearly relating to the annual cycle of the year. The occurrence of the Anglo-Saxon compounds ymbren-tid ("Embertide"), ymbren-wucan ("Ember weeks"), ymbren-fisstan ("Ember fasts"), ymbren-dagas ("Ember days") makes this etymology quite certain. The word imbren even makes it into the acts of the "Council of Ænham"^[6] (1009): jejunia quatuor tempora quae imbren vocant, "the fasts of the four seasons which are called "imbren'".^[7] It corresponds also with Pope Leo the Great's definition, jejunia ecclesiastica per totius anni circulum distributa ("fasts of the church distributed through the whole circuit of the year").

However, others maintain that the term is derived from the Latin quatuor tempora, meaning "four times" (a year), while folk etymology even cites the phrase "may ye remember (the inevitability of death)" as the source. J. M. Neale's Essays of Liturgiology (1863), Chapter X, explains the etymology:

"The Latin name has remained in modern languages, though the contrary is sometimes affirmed, Quatuor Tempora, the Four Times. In French and Italian the term is the same; in Spanish and Portuguese they are simply Temporas. The German converts them into Quatember, and thence, by the easy corruption of dropping the first syllable, a corruption which also takes place in some other words, we get the English Ember. Thus, there is no occasion to seek after an etymology in embers; or with Nelson, to extravagate still further to the noun ymbren, a recurrence, as if all holy seasons did not equally recur. Ember-week in Wales is Welsh: "Wythnos y cydgorian", meaning "the Week of the Processions". In mediæval Germany they were called Weihfasten, Wiegfastan, Wiegefasten, or the like, on the general principle of their sanctity.... We meet with the term Frohnfasten, frohne being the then word for travail. Why they were named foldfasten it is less easy to say."

"Quattuor tempora" was rendered into Irish quite literally as Laethanta na gCeithre Thráth, meaning "the days of the four times", and into somewhat archaic English as "Quarter tense".

This is a beautiful setting of the Introit text composed by Jacquet de Mantua, a name new to me.



This is actually a shorter take on Psalm 70/71, including only vv. 7-8 (note that the numbering system for the verses in this Psalm varies from translation to translation):

Repleatur os meum laude, ut cantem gloriam tuam, tota die magnitudinem tuam.
Ne projicias me in tempore senectutis; cum defecerit virtus mea, ne derelinquas me.

Let my mouth be filled with your praise, and I will sing a hymn to your glory and magnificence all day long.
Do not reject me in the time of old age; do not abandon me when my strength fails.

The YouTube page lists this group of singers:

Paolo Costa y Claudio Cavina, contratenores.
Fabio Fùrnari y Giuseppe Maletto, tenores.
Marco Scavazza, barítono.
Marcello Vargetto, bajo.
Delitiae Musicae.
Marco Longhini.

And Wikipedia says this about de Mantua in its intro; there is more at the link:

Jacquet of Mantua (Jacques Colebault, dit Jachet de Mantoue) (1483 – October 2, 1559) was a French^[1] composer of the Renaissance, who spent almost his entire life in Italy. He was an influential member of the generation between Josquin and Palestrina, and represents well the transitional polyphonic style between those two composers.