Uruguay is a temperate climate, with no real dry or wet season. It can be fairly cold at certain times of the year but in general it’s ideal for slotting in to a Brazil or Argentina itinerary.
All you really need to know is that its winter months (May to September) are quite windy and cold, so not recommended. March on the other hand is a great time to go as this is when they hold the Fiesta de la Patria Gaucho – a celebration of Uruguay’s cowboy culture, featuring rodeos, parades, and folk music.