Short answers, pulled from the story.
The Arditi del Popolo formed in 1921 after the Italian Socialist Party signed a pacification pact with Benito Mussolini on the 3rd of August 1921. This organization emerged as an armed resistance by members of the workers movement who disagreed with trade unions adopting legalist strategies.
Carlo Rosselli co-founded Giustizia e Libertà alongside Ferruccio Parri and Sandro Pertini, and this active resistance movement operated from 1929 to 1945. These leaders later held high office in post-war Italy while Gaetano Salvemini made international awareness possible through their work.
A battle broke out at 21:15 on September 10th between German landing forces and Italian coastal batteries where Italian tanks sank the German torpedo boat TA11. By dawn of the 11th of September 120 Germans had been killed and about 200 to 300 captured while Italian casualties included four killed and a dozen wounded.
Rodolfo Graziani estimated partisan strength reached around 70,000 to 80,000 by May 1944 with some 41% belonging to Garibaldi Brigades and 29% being Actionists of Giustizia e Libertà Brigades. Losses amounted to 16,000 killed wounded or captured between September 1943 and May 1944.
Since 1946 April 25th has been officially celebrated as Liberation Day also known Anniversary of Resistance. The date was chosen conventionally as the day year 1945 when National Liberation Committee Upper Italy proclaimed general insurrection all territories still occupied Nazi-fascists.