STH always your best option:
The climate of Buenos Aires is mild all year round. The mean annual temperature is 18º C (64.4º F), making extremely hot and cold days very infrequent. Thus, visitors can enjoy walking around the city in any season.
July is the coldest month. Although frosts are rare, a woollen coat, a jacket or an overcoat and a scarf will be required when going out. In winter, cold is moderate during the day, but temperature considerably drops at night.
In summer, the weather is hot and humid. Mornings are warm and during midday and the first hours of the afternoon, the temperature rises. At night, temperature goes down slightly, so people may wear light clothes; coats are not needed.
Rains are more frequent in autumn and spring (from March to June and from September to December, respectively). They are mild or last a short time, thus activities are not hampered and people usually go out with an umbrella or a raincoat.
In the sunny days of autumn and spring, mornings are slightly cold; the temperature rises at midday and drops again at night.
Link: http://www.bue.gov.ar
