Twenty-seven kilometres west of Zurich, Baden is usually a quiet spa town. The Swiss flag above its castle ruin flutters in the breeze, the locals soak their feet in a strategically placed footbath along the Limmat River that bubbles with the famous thermal water for which Baden is named, and the street sweeper buzzes down already tidy streets where everything from shop windows to silver rubbish bins shine – streets where no geranium would dare appear askew.

But every 10 years, Swiss-German rap music blares across the town until 5am, making even concrete buildings vibrate. Every 10 years, thousands of festival attendees toss beer bottles and sausage sticks into the streets. And every 10 years, it’s not only OK to shriek all night long, but it’s encouraged – even if, normally, you can’t even recycle a few cans during lunchtime without a stern lecture about destroying the peace.

Because every 10 years, Baden hosts a raging 10-day party that defies what it means to experience Switzerland. Firstly, there are crowds – something that a country of only eight million doesn’t usually have a lot of. But Baden, a town of 16,000, draws a million visitors to its festival. When compared for population size, that’s as if 525 million people (a number greater than the entire population of the United Kingdom) descended on London for an event.

The 2017 edition takes place from 18-27 August.

Going to Baden for entertainment has a history. In the Middle Ages, Baden’s spas were a popular escape for dignitaries. And during the Reformation, many Protestant Zurichers found themselves fleeing to Catholic Baden for the kind of elusive fun that only Catholics could have back then.

Every 10 years the Swiss town is filled with very un-Swiss revellers (Zurich Tourismus)

But the real party began in 1847, when Switzerland’s first train route opened. The “Spanischbrötli-Bahn,” which went from Zurich to Baden, made it easy for Zurichers to come to “crazy” Baden to eat sweet pastries (like Baden’s specialty, Spanischbrötli, for which the train was named), sing, and let loose at the thermal spas. If you spoke of Badenfahrt in the 19th century, you were riding the rails to Baden to have a good time.

But numbers and a little Baden history tell only one part of the story; the pure emotion of today’s Badenfahrt participants tells the other. In fact, Badenfahrt is more than a festival – Badenfahrt is a transformer. The festival transforms not only Baden, but also the people who attend it.

The Swiss aren’t known for being emotional – or loud. A private, quiet culture, the Swiss are usually so aloof that overly expressive Americans who want to be culturally appropriate have to train themselves not to smile at people. English speakers in Switzerland also find themselves redefining normal speaking volumes, usually after receiving a comment from a Swiss like: “So loud. Why so loud?” as I once did when conversing with a fellow American at a tram stop.

Foreigners – and even Swiss people – get to know Swiss people slowly. Year after year, you peel away at their outer layers, like you’re peeling an onion – trying to find the life story that other people might tell you in the first hour of acquaintance. But that’s the magic of Badenfahrt: it can do in a few hours what years can do when it comes to getting to know the Swiss.

I speak from experience – it was the first day of Badenfahrt in August 2007 when I learned the first name of my Swiss neighbour. Before then, we'd been on formal, last-name terms for over a year. Badenfahrt changed all of that. In one festival day, after a re-introductory handshake, we went from the formal “Sie” to the casual “du”.

This year’s Badenfahrt motto is “Versus.” And it couldn’t be more appropriate. It means “in contrast to” and it invites attendees to experience Baden’s opposites: the noisy festival versus the idyllic Limmat River, the modern industrial side of Baden versus the cobblestoned old town, the Swiss people at the festival versus the Swiss people not at the festival.

Polite as they are, the Swiss tend to relax most when given an organised reason to do so. Badenfahrt provides that reason. The festival features parades, fireworks, carnival rides, several entertainment stages, and that oh-so-Swiss trust that allows a million people into an unfenced festival area knowing that they’ll have all bought tickets anyway.

The festival features hundreds of creatively themed food structures representing everything that Swiss industriousness is capable of. Forget crappy food tents; this year, the main food structure near the Schlossbergplatz will cost CHF100,000 (£80,927) to build. They'll build replica Greek temples, along with sand beaches, the Eiffel Tower, and Japanese Gardens. Give the Swiss some scaffolding, and they’ll give you the Taj Mahal – even if its only purpose is to serve sausage for 10 days.

The streets are rammed for the Badenfahrt (Brian Opyd)

Aside from Zurich’s Street Parade, Badenfahrt is the festival to witness the Swiss as you never have before. To experience Badenfahrt is to experience the Swiss at their most fun-loving and free.

Until of course, the clock strikes midnight on the 10th day of the festival and street sweepers arrive on the 12th ring to start cleaning up the mess.

Travel essentials

Getting there

British Airways, easyJet, and Swiss, all fly from London to Zurich. From there, trains to Baden take 34 minutes and cost CHF8 (£6.50). A taxi costs about CHF120 (£97) and takes 30 minutes.

Staying there

For a true cultural experience (and a good price), forget the hotel and stay at Frau Meise B&B. Doubles from CHF140 (£113).

Otherwise, La Cittadella, The Residence Baden offers brand new studios in the heart of the old town. Doubles from CHF170 (£138).

More information