My husband, 8-month-old daughter and I had flown into the city just a couple of hours earlier, and as we wandered along the Limmat River, we heard three booms that sounded as though they had come from a cannon.
We weren't sure what it was - or why - but as a crowd began gathering at the Weinplatz, an open plaza just off the river, we joined in.
That's when we noticed that mixed in with the Swiss teenagers in jeans and coats were some people decked out in period garb.
Men wore knickerbockers, bright stockings and full-length cloaks.
Women wore hoop skirts, dirndls and dresses with aprons.
The cannon sound had come from a barge that was quickly closing in on a dock near the Weinplatz, with passengers aboard in outfits that would have been perfectly appropriate for one of the Crusades.
This, we learned, was Sechselauten - the city's singular celebration of the arrival of spring.
Each April, the people of Zurich celebrate the end of winter with a weekend of balls, parades and the ceremonial burning of a snowman effigy.
And, as a brass band began to herald the barge's arrival, we realised we had found ourselves, by sheer luck, in the middle of it all.
As we wandered the city, we basked in the warm sunshine with a mix of tourists and natives who had come out for Sechselauten.
Pastry shops marked the occasion with special snowman treats.
Flowers were everywhere.
For the non-German speaker, information on the festival was scant, which made it both a challenge and an adventure.
Although there's a detailed German-language website about the event, all the tourist information office had for us was a one-page English explanation.
"Every year the guilds of Zurich and the townspeople celebrate with great gusto their traditional spring festival," the English material said.
So, like our choice of Zurich itself, our Sechselauten was, at least at first, filled with serendipity.
A walk along the shores of the Lake of Zurich, to show our daughter the swans, yielded a small street fair nearby.
There were carnival rides, stands selling wurst and a towering Ferris wheel.
Our hike to visit the Lindenhof, where old Zurich's Roman castle once stood, meant we also found (besides more wurst) Chapeau, an outdoor music festival with performers as diverse as brass bands and full rock.
The juicy wurst, which was served with a fist-sized roll and a dollop of spicy mustard, was our culinary mainstay for the three days we were in Zurich.
By Sunday, our second day, we had deciphered Sechselauten's schedule enough to find a place along the Bahnhofstrasse, the city's main shopping strip, to watch the children's parade.
A few minutes after 2pm, the parade began with music and groups of schoolchildren, each representing an era of Zurich's history, from Roman times through to World War 2.
Little boys were dressed up as knights, girls wore Marie Antoinette wigs that tilted over their delicate features, and an eager crowd cheered the marchers on, sometimes greeting them by name.
The word "Sechselauten" translates as "the 6 o'clock hour ringing of the bells" - a nod to the time when the work day was over for guild members in medieval times.
And Sechselauten traditionally concludes at 6pm on Monday with the burning of the Boogg - that snowman effigy, whose head is filled with explosives.
Folk wisdom holds that the time between the lighting of the Boogg's pyre and the explosion of its head is a clue to what the summer will be like. (A quick burn signifies a long, warm summer; a drawn-out fire means a cold and dreary one.) It took a little more than 26 minutes last year, a fairly swift and promising showing.
