Chopin composed the so-called Nocturne in C-sharp minor in 1830, and dedicated it to his older sister, Ludwika Chopin: “For my sister Ludwika to practice before she takes on my second Concerto.” He did not give it a name and did not call it a nocturne. Ludwika, however, referred to it as “Lento, of a nocturne character” when she compiled a list of her brother’s unpublished compositions much later.
The piece was not published until 1875, twenty-six years after the composer’s death, together with three mazurkas, under the title Lento con gran espressione, from its tempo. It is sometimes also known as Reminiscence. I could not find out when it got officially counted under the nocturnes as number 20.