The patron saint of Somma Lombardo is Saint Agnes, celebrated on 21 January.
In summer, the Visconti di San Vito Castle hosts the «Festa Medievale», re-enacting Ottone Visconti's victory over the Della Torre troops in the Battle of Desio: a clash fought precisely on 21 January 1277, the same date as the patronal feast, and from which the Duchy of Milan originated. The municipal territory also holds the Sanctuary of the Madonna della Ghianda and the Volandia Park and Museum.