This report is a slight variant of one printed in the Gazette d’Utrecht on 20 Sep 1765 (Neue Folge, 5). That story, under the dateline 19 Sep, states that Mozart’s performance had taken place “yesterday” (“hier”), thus establishing the date as 18 Sep. The version above from the Journal encyclopédique, a monthly journal (where writing “yesterday” would have been unclear), revises the wording to state explicitly that the concert took place on 18 Sep.
The “Prince Stadthouder” was William V, Prince of Orange (1748–1806). William was just three years old when his father William IV, the hereditary “Stadtholder” of the Dutch Republic, died in 1751. (The “stadtholder”—this is the standard English spelling—was a kind of chief magistrate, but when the title became hereditary under William’s father, its holder was in effect the monarch.) A series of regents governed in young William’s stead until he reached his majority. His official installation took place when he turned 18, on 8 Mar 1766; Mozart composed his variations K 24 and K. 25, and the Galimathias musicum, K. 32, in honor of that event. Mozart’s set of Six Sonatas for Keyboard with the Accompaniment of Violin, K. 26–31, were dedicated to William’s older sister Carolina, Princess of Nassau-Weilburg, the last of William’s four regents (see also our entry for 28 Feb 1765); in fact, it was her strong desire to hear the Mozart children that brought the family to the Dutch Republic, which had not been on their original itinerary (see Leopold Mozart’s letter of 19 Sep 1765 to Lorenz Hagenauer, Briefe, i:201). At the time of Mozart’s performance in Sep 1765, William was still underage and had not yet officially assumed the title of Stadtholder.
The first Dutch translation of Leopold Mozart’s Versuch einer gründlichen Violinschule, published in Apr 1766, is also dedicated to William V (see our entry on that translation).
The Mozarts reached The Hague, the seat of government in the Dutch Republic, on 10 or 11 Sep 1765 (on the timing of this portion of the Mozarts’ trip, see our entry for 9–10 Aug 1765). But the following day Nannerl became seriously ill: as Leopold described it in his letter to Hagenauer, she “hat einen sehr starken Brust Cartharr” (“had a very strong chest catarrh”; Briefe, i:203). The illness worsened the following month to the point that she was given extreme unction on 21 Oct, and it was only around the end of the first week of Nov that she had recovered to the point that she could get out of bed (Halliwell 1998, 100–101). This is the context for Wolfgang’s performance for William and (presumably) Carolina on 18 Sep 1765: Nannerl did not take part, because she was ill. Shortly after her recovery, Wolfgang himself fell seriously ill on 15 Nov, recovering only in mid Dec (see Leopold’s letter to Hagenauer of 12 Dec 1765, Briefe, i: 211).
An identical report to the one in the Journal encyclopédique (including the spelling error “Woffgang”) appeared in the Gazette des gazettes, ou Journal politique in the “deuxième quinzaine” (second half) of Sep 1765 (69–70). At that time, both the Gazette des gazettes and the Journal encyclopédique, were printed in the city of Bouillon in the southern, French-speaking half of present-day Belgium. Jacques Renéaume de la Tache (1725–1796), editor of the Gazette des gazettes, also contributed to the Journal encyclopédique, published by Pierre Rousseau (1716–1785); this connection explains the presence of the identical text in both newspapers.
Mozart’s performance at the Stadtholder’s court is also mentioned, although without a specific date, in a brief report in the Oprechte Haerlemse Courant on 19 Sep 1765; see our entry on that report.