Un giorno di regno, ossia Il finto Stanislao (A One-Day Reign, or The Pretend Stanislaus, but often translated into English as King for a Day) is an operatic melodramma giocoso in two acts by Giuseppe Verdi to an Italian libretto written in 1818 by Felice Romani.

Un giorno was first of the three, but he wrote the piece during a period when first his children and then his wife died and its failure in 1840 caused the young composer to almost abandon opera.

Along with the critics, Verdi acknowledged that the failure was partly due to his own personal circumstances, since his two children and his wife Margherita Barezzi had died, in 1838, 1839, and 1840 respectively,[4] all during the period leading up to and during its composition.

[5] In fact, in summary, Budden notes that "by the side of Donizetti's L'elisir d'amore or Don Pasquale, it cuts a clumsy figure".

[9] In the UK, the premiere took place on 21 March 1961 in Italian by the Impressario Society at St Pancras Town Hall conducted by Hans Ucko.

[16] The Polish monarch, King Stanisław Leszczyński, a historical figure during the War of Succession, lost his throne after the Saxon invasion at the Battle of Poltava in 1709.

The opera is set in 1733 when Stanislaw returned to Poland leaving a French officer, the Cavaliere di Belfiore, to impersonate him in France.

Another undesired marriage involves Baron Kelbar's niece, the Marchesa del Poggio, a young widow who is in love with Belfiore.

Knowing of the Marchesa's imminent arrival and concerned that she might reveal his false identity as the King, Belfiore writes to Stanislaw and asks to be released from his commitment.

Likewise, he pretends not to recognize her, but she is determined to test him by proclaiming her love for the Count: "Grave a core innamorato...Se dee cader la vedova".

Scene 2: The Garden of Kelbar's castle Giulietta is alone with her attendants and expresses unhappiness in having to marry an old man: "Non san quant'io nel petto...Non vo' quel vecchio".

Scene 3: The gallery of Kelbar's castle Maintaining his role as the King, Belfiore makes the Treasurer an offer of advancement which would include marriage to a rich widow.

Scene 1: The gallery of Kelbar's castle Following the "King's" pronouncement, the servants are mystified and they sing a carefree chorus which leads to Edoardo seeking their support and announcing his hope of still being able to marry Giulietta: "Pietoso al lungo pianto...Deh lasciate a un alma amante".