This Saturday, May 1st, is a holiday in Italy (their Labor Day) and is celebrated in Rome every year with a big free concert at San Giovanni. The lineup is mostly made up of Italian artists and the show will go on all day and even well into the night.
If you are not familiar with San Giovanni, it's just a couple stops from Termini on the Metro A, and has a lot of buses pass by it. Google map it here on our main page if your unsure. When you get off the Metro, just go under the old Roman aqueduct and you're there. It will probably be absolutely packed with people (that's last year in the picture above), so I am sure you won't have trouble finding it.