The origin of the feast dates back to the reign of emperor Augustus: in 18 BC he instituted the Feriae Augusti to mark the end of summer harvest and provide a day of rest for all. As frequently happen the pagan date now coincide with a religious one.
Ferragosto is one of this clue days along the year when we party and look for absolute leisure with our beloved ones.
The week around August 15 is when you easily find many commercial sites closed for vacation as traditionally August is the month we select for summer vacations.
On August 15 in Rome, many major sites are open except the Vatican museum.