The San Telmo Fair (Feria de San Telmo) is the biggest fair in Buenos Aires. Its held every Sunday on Avenida Defensa in San Telmo (click here to see the location).
The street is lined with 1000s of market stalls, live bands, tango dancing, food stalls, street sellers, antique stores and touristic mementos. Its a great day out and certainly one of the top things to do in Buenos Aires.
Avenida Defensa was originally one of the oldest streets in Buenos Aires that leads upto the government house (casa rosada). Its also home to many famous churches, plazas and buildings, such as Plaza Dorrego, Basilica de San Francisco, Convento de Santo Domingo and Parque Lezama. The story about Parque Lezama is during the yellow fever in Buenos Aires in 1852, they put the 1000s of dead bodies underneath Parque Lezama. Perhaps its of interest to some people or a place to avoid for others.