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At the funeral of the tenor Adolphe Nourrit in Paris in 1839, Chopin made a rare appearance at the organ, playing a transcription of Franz Schubert's lied Die Gestirne. On 26 July 1840 Chopin and Sand were present at the dress rehearsal of Berlioz's Grande symphonie funebre et triomphale, composed to commemorate the tenth anniversary of the July Revolution. Chopin was reportedly unimpressed with the composition.

It was in commemoration of the tenth anniversary of what?
the July Revolution.