Believe it or not we went to a great jineteada in San Isidro about this time of year a couple of years back, you could keep an eye on the events page here, they had one in July:
http://sanisidro.gob.ar/ai1ec_event/jineteada-y-baile-por-el-9-de-julio/?instance_id=241
Really authentic and a great asado but take your own knife!