Up to 63% of the entire population of Italy is on vacation for at least some of the month of August. Most government offices are shut, as are many independently-run businesses such as small grocery stores, family restaurants, etc. – but if you’re a tourist this shouldn’t affect you too badly as most places that cater to visitors remain open.
For most of the month, anything at the beach or related to domestic tourism such as parks, beach restaurants, hillside attractions, mountain resorts, etc. will be open – but extremely crowded, as they will be full of vacationing Italians.

However August 15th is Ferragosto, the peak of this holiday and on that day things really are shut. It’s a time when extended families get together and spend time with each other, at least as big as Christmas, so you will find that many things that were open for August are closed on the 15th. Supermarkets are usually closed, as are many of the restaurants that remain open during other days in August.
To prepare yourself, if there’s anything you need on that day, join the crowds stocking up. Though remember that many Bangladeshi- and Chinese-run convenience stores will remain open for esssentials.
We recommend that you do some research beforehand to see which restaurants will remain open on that day so you don’t miss out – though obviously hotel restaurants etc. will be open. Here’s a list of restaurants that will definitely be open in August, and some even open on Ferragosto itself.
Additionally in the Christian calendar this is the Feast of the Assumption so you will find that many churches are closed for services, including the Pantheon at 5pm on the 14th August, 10.30am on the 15th, and it will close at 1.30pm that day. St Peters Basilica will be open but celebrating various masses including the papal Angelus.
The Vatican Museums are shut on the 14th and 15th August. The Colosseum, however remains open every day in August.
Pro tip: for some light relief, the Cinecittà World theme park is open all day on Ferragosto, and is delightfully empty that day, meaning you will hardly have to line up for any rides. However you will need to make your own travel arrangements to get there – perhaps renting a car the day before.