Medellin's climate barely changes. It sits at 1,500 metres in a valley near the equator, which gives it an almost constant 20–24°C year round — the "Eternal Spring" nickname is earned, not invented. The drier months are December through February and July–August; the rest of the year brings short afternoon showers.
The standout event is the Feria de las Flores in early August: a ten-day flower festival that culminates in the Desfile de Silleteros, a procession of farmers from Santa Elena carrying enormous flower arrangements strapped to their backs. If you come for one thing, come for this.
Other worthwhile dates: Colombiamoda fashion week in July, the International Poetry Festival in July (one of the largest in the world), the Jazz festival in September, and Alumbrados — the Christmas lights along the Medellin River, running the whole of December.