Hundreds of people decked out in fine, vintage black clothes will have a picnic in downtown Ottawa Sunday afternoon.

But they won't be told which park to go to until the last minute.

The chic, pop-up picnic concept that started in France has taken hold across the country.

In Ottawa, it got started last year as Harvest Noir, and has roughly doubled in size this year, organizers say.

When she's not teaching, Andrea Emery is pinning fabric.

"I am sewing a vintage 1800s bustle skirt, and a frock coat for a gentleman, and I'm more excited about this than about anything else right now," she said Friday.

Emery didn't go to last year's Harvest Noir. But she envied the photos of picnickers dressed in elegant black, and she likes the idea of hundreds of people descending on a surprise location.

"Ottawa seems to be such a sleepy little town, so hearing about this seems very exciting … very underground," Emery said.

She's not alone. The $50 tickets, that cover venue rentals and other costs, are sold out.

Organizer Greg Searle bought sparklers for 1,500 people, double the number who attended last year.

He said people like the mystery and ambience.

"It's not like going to a theatre and sitting down and being performed at. The whole thing is a performance art where everybody's the artist," he said.

After the picnic Sunday, the partiers will parade through the downtown and have a dance indoors.

Emery is surprised so many people want to take part in Harvest Noir.

"Fifteen hundred? Where are we going to picnic? Fifteen hundred of us! That's crazy. I'm praying for sunny weather, because right now it looks like it's going to be cloudy. I don't think that's going to dampen our spirits. I have two big black umbrellas.