"on the day of Gambetta's funeral"

Léon Gambetta by Léon Bonnat
Public DomainLéon Gambetta by Léon Bonnat
Léon Gambetta (1838-1882) was a French statesman who became prominent after the Franco-Prussian War (see bookmark to p. 104, "I can remember them in '70"). He died on 31st December 1882, so his funeral was presumably early in January 1883; but Proust plays a bit fast and loose with dates in the Search so this does not necessarily provide a particularly useful signpost in time.