The passage transcribed and translated above is found in a letter dated 19 Aug 1771 from Count Carlo di Firmian in Vienna to the writer Giuseppe Parini in Milan (on Firmian, see our entry for 4 Apr 1770; on Parini, see our entry for 17 Oct 1771). At the beginning of that year, Empress Maria Theresia had informed Firmian, Habsburg minister plenipotentiary and de facto ruler of the Duchy of Milan, that she wished to commission a serenata as part of the festivities for the wedding in Milan of Archduke Ferdinand and Princess Maria Beatrice d’Este that coming autumn; the serenata would be in addition to a new opera that had already been planned (that opera would become Il Ruggiero by Johann Adolph Hasse). The empress had left it to Firmian to choose the poet and composer for the serenata, and he selected Parini and Mozart for the commission that became Ascanio in Alba (see our entries for 9 Feb 1771, 7 Mar 1771, 9 Mar 1771, and 17 Oct 1771).
In a letter dated 20 Apr 1771, Firmian had written to Count Johann Wenzel Sporck in Vienna that Parini had offered a choice of two subjects for the serenata: Ascanio in Alba and Le nozze d’Atalanta (for this and the following, see Sommer 1981, 496; on Le nozze d'Atalanta, see also our entry for 17 Oct 1771). On 2 May, Sporck had replied that the empress preferred Ascanio, and Parini was informed of her choice on 23 May. Three months later, on 5 Aug, Firmian wrote to Parini from Vienna to acknowledge receipt of the finished libretto, saying that he had given it to the empress. Thus Parini did most of the work on the libretto for Ascanio in Alba between the end of May and the end of July 1771.
In the passage transcribed above, written two weeks after acknowledging receipt of the libretto, Firmian informs Parini that it has been returned, and that he is forwarding it immediately to Parini along with the letter. In the revised version of the letter as it was presumably sent, Firmian does not say whether the empress herself commented on the libretto, but he writes approvingly of it; his honorific “L[oro] A[ltezze] R[eali]” (“Their Royal Highnesses”) refers to Archduke Ferdinand and the Princess. But Firmian’s original draft contains the cancelled phrase “e godo di poterle dire ch’essa ha avuto la sorte d’incontrare il Reale suo gradimento” (“and I am happy to be able to tell you that it had the fortune to meet Royal approval”). Why Firmian decided to remove this direct indication of the empress’s approval is unknown. Firmian himself asks only that the libretto be shortened a bit, in order that the whole performance not last more than two hours, in order to allow time for events afterwards. Firmian writes that Mozart is already on his way from Salzburg to Milan—Wolfgang and Leopold had departed Salzburg on 13 Aug and arrived in Milan on 21 Aug—and he asks Parini to make final changes to the libretto as quickly as possible, so that Mozart will have sufficient time to compose the music.
According a letter dated 24 Aug from Leopold Mozart in Milan to his wife in Salzburg, the libretto had not yet arrived from Vienna:
[...] Ich muß dir sagen, daß die Poesie von Wienn noch nicht angelangt ist. Man erwartet sie mit entsetzlicher Begierde, indem man sonst mit verfertigung der Kleider und ausziehrung, veränderung etc: des Theaters nicht kann fertig werden. den 15ten octobris wird der Erzherzog in Mayland eintreffen [...] [Briefe, i:431]
[...] I must tell you that the poetry has not yet arrived from Vienna. It is awaited with terrible eagerness, for without it the manufacture of the costumes and decorations, scenery etc. for the theater cannot be completed. The Archduke will arrive in Milan on 15 October [...]
On 31 Aug, Leopold wrote to his wife that the libretto had arrived:
[...] Die Poesie ist endlich angelangt, der Wolfg: hat aber noch nichts als die overtura gemacht, nämlich ein etwas Langeres Allegro, dann eine Andante, welches gleich muß gedanzt werden aber nur mit wenigen Personen, dann anstatt dem letzten allegro hat er eine Art von Contradance und Chor gemacht, so zu gleich gesungen und gedanzt wird. Nun wird es dieses Monat durch zimmlich Arbeit geben. [...] [Briefe, i:433]
[...] The poetry has finally arrived, but Wolfgang has so far written nothing except the overture, namely a somewhat longer Allegro, then an Andante, which must at the same time be danced, but with only a few people, then instead of the final Allegro, he has written a kind of contredanse and chorus, that must be sung and danced at the same time. Now there will be very much work through the whole month. [...]
In a letter to his wife a week later, on 7 Sep, Leopold clarified that Parini had kept the libretto for several days after it had arrived to make changes, and Wolfgang had received it just two days earlier, that is, 5 Sep:
Wir haben itzt den Kopf mit anderen sachen voll: indem die Poesie späth angelangt und noch zur änderung ein und anderes bis vor 2 Tägen in den Händen des Poeten geblieben. Es wird hoffe gut ausfall: allein der Wolfg: hat nun die hände voll zu schreiben, da er auch das Ballet, so die zwey act oder theile mit einander verbindet, Komponieren muß. [Briefe, i:435]
We now have our heads full of other things: for the poetry arrived late, and then remained until two days ago in the poet’s hands for one change and another. I hope everything will turn out well: but Wolfgang now had his hands full with writing, as he also has to compose the ballet that connects the two acts, or parts.
Just six days later, Leopold wrote that Wolfgang had finished all the choruses, as well as all accompanied and simple recitatives; he also wrote that they had attended the first dance rehearsals that same day, 13 Sep. In a postscript to Leopold’s letter of 21 Sep, Wolfgang apologized to Nannerl for writing so little because his fingers were sore, remarking that he had just two arias left to write (Briefe, i: 439). He finished composing the serenata on 23 Sep, apparently having composed everything but the overture in just eighteen days. The first choral rehearsals took place on 27 Sep, and the first full rehearsal on 28 Sep, with further rehearsals on 4, 8, 11, and 14 Oct. The first performance of Ascanio in Alba took place in the Regio Ducal Teatro on 17 Oct 1771 (see our entry for that date).
Whether any of the changes that Parini made to the libretto were in response to Firmian’s request remains unknown. Although a complete scenario for the serenata survives in Parini’s hand (see Silvestro 2018, 178–180), no other autograph manuscripts of the poem are known to survive. Modern complete recordings of Ascanio in Alba last over two and a half hours. Even if the performances in 1771 ran longer than Firmian’s hoped-for two hours, there were events afterwards: according to Parini’s own description of the wedding festivities (see our entry for 17 Oct 1771), the performances of Ascanio in Alba on 27 and 28 Oct were followed by masked balls.