The Medellín region is known for its flowers (and is a major exporter to the USA), and every August the city pulls out all the stops for its 66-year-old Feria de las Flores, the highlight of which is this Desfile de Silleteros (Flower Growers' Parade). There's plenty else on the menu, too, including music concerts and a festival, other cultural activities, and sports events like the Lights Park cycling event downtown. I attended the festival several years ago and had a great time!

(And by the way, whatever time of year you visit, should also work in a visit to a flower farm in nearby Santa Elena, the hub of the industry.)