Professor Barry Cooper indicates that at this time Beethoven at last begins serious work on Piano Sonata no. 31, op.110. He had been promising the next two sonatas to Schlesinger, his publisher in Berlin, for over a year at this point. A few sketches for op.110 appear in the Artaria 197 sketchbook from 1820, and a few more sketches were made in June of 1820, but this is when the piece really starts to take shape. It will occupy him until Christmas, interspersed with his endless reworking of the Missa Solemnis.
It is probably no coincidence that this burst of energy coincides with Beethoven’s rapidly recovering health, hastened by his move to Baden.