Made a mistake - the fair is in San telmo which is a barrio within BA. Everyone in the city will know of it as it is quite popular. It pretty much runs all day until about 5 or 6PM. The street also has a few art galleries many antique shops. I wish I new something about it as the deals are probably pretty good compared to US.
If you like red wine, you will need to drink at least one or two bottles of Malbec. They are really great and I have started shopping for them back home in CA.
Make sure that you ask at you hotel regarding the distance to Calle Florida before walking. I think it may be a couple of miles. The metro is also pretty good if it is nearby. However, with cabs so inexpensive it doesn't make too much sense.