Ferragosto. More precisely, the national holiday is on August 15th, which began its life as a religious holiday, the Feast of the Assumption of Mary in the Roman Catholic Church, "morphing" into a holiday/vacation with the evolution of Italy's "beach culture" in the early 1960s. So much for religion in "Catholic" Italy...
History aside - and because Best of Sicily prides itself on being the kind of Sicily travel guide that presents accurate information instead of clichés and platitudes - it's worth considering what you should expect this week, when half the country is on vacation. After thinking about how ridiculous it is for tens of millions of Italians to choose the same week for their holiday, you'll want to think about what to avoid.
Beaches will be crowded, especially close to the largest cities. Mondello, outside Palermo, is the worst in Sicily.
Many restaurants will be closed all week, especially in the city. Most stores will be closed Wednesday, the 15th. All banks and public offices will be closed that day. Check out the Sicily weather forecast to see how hot it'll be. With any luck, you can find a place in the shade and a nice gelateria.
You've been warned.