Why it is called 91 days if there were only 12? So, basically the question in the title. I thought it had something to do with how long it took for Angelo to complete his revenge but I'm not too sure.
The 90 days signify the days left until the opening of the Opera House, which was mentioned in the wedding episode 2 by Frate, who tells them that Don Gallasia decided to attend the opening show. The 91 days refers to the time in which his revenge takes place since arriving.