A patriot to the core, he devotes his entire life to promoting Swiss values through the beauty and grandeur of the Swiss landscape. He is active in the political life of his native city, espousing moderate liberal ideas and strongly opposing the governmental conservatives of the Restoration.
Diday is the first Genevan painter to prize the Alps as a motif; from 1829 on he is an art teacher himself, numbering amongst his students Alexandre Calame, who becomes both a friend and a rival landscape painter. Thanks to these two artists, Genevan landscape painting comes to be known throughout the whole of Europe. They exhibit in Paris, Brussels, Vienna, Turin; their works are awarded prizes, admired and acquired by collectors and the crowned heads of Europe.
A politically committed artist, Diday looks to create a free drawing school in Geneva and offers the Arts Society money for organising an art competition, whose laureates will eventually include Hodler. Diday also bequeaths two buildings to the City of Geneva, the income from which is to be put each year towards purchasing art works by Swiss artists.