The official parade which begins the carnival season usually takes place around January 31st -usually on a Friday. This parade takes place on the main street of the city: 18 de Julio, from Plaza Constitucion until La Intendencia (City Hall). There is a second parade, just for "Candombe" (one of the different types of carnival groups) which usually takes place one week after the main parade, in a nearby neighbourhood.

The official carnival competition begins just after the main parade takes place. This competition takes place in an open air theatre called "Teatro de Verano", next to Playa Ramirez, on the coast. The competition lasts at least until the end of February, sometimes even mid-March depending on the rain, which tends to cancel a few shows during February.

Although there are several other open air theatres around the city, were you can see shows every night, I highly recommend you check the shows at the "Teatro de Verano" as this is the official competition, with the juries and everything, so the groups try their best to score the highest points.

I hope this info helps. Have fun in Uruguay!
Summer time / Carnival season is the best time to visit this place.