Every first day of the month is possible to visit for free the lovely casino ("little house") of the Pallavicini-Rospigliosi palace, by the Quirinal Hill.
The place is interesting to visit, first of all, because it's a lovely example of the tastes of that fox Cardinal Scipione Borghese (if Galleria Borghese wasn't enough, I mean!) and because of the special feature of the lovely and popular Aurora frescos attributed to Guido Reni.
As pictures were forbidden in the area, I just grabbed those from the internet :D Credits for this image goes to Wikipedia!
I wouldn't talk much about the fresco, as everything is said in the Wikipedia article already, but just a little mention to the breataking facade of the tiny pergolato that hosts the masterpiece is worth a note.
The picture, by the way, comes from Tripadvisor:
The massive decoration sports a tendency of the villas of the period, showing off collections of Roman antiquities blended into the modern architecture and decorations. The plasters come from ancient Roman sarcophagi, and their common theme is the immortality of the soul; the same decorations are repeated inside the casino in the form of frescoes by the creative hand of Antonio Tempesta, who depicted the Triumph of Fame and the Triumph of Love... If you happen to be in Rome on the first day of the month, consider a walk around here!