Saturday afternoon, Somerville Avenue closed to cars from Church Street to the Union Square municipal lot, and roughly 30,000 people filled in the gap. Fluff jousting on the Shenanigans Stage. A cover band on the Fluff 'N Jam Stage. Free bike valet at two spots along the avenue, because organizers have spent two decades telling people not to bother driving in. Arthur Read, aardvark and Somerville's most durable ambassador via GBH, worked the crowd for photos. The theme this year was "Legally Fluff: A Toast to Union Square," a nod to the fact that a festival born in 2005 just turned old enough to drink.

That's the fun part, and if you were there, you already know it. What's more interesting is what the festival was actually built to do, and whether, 21 years later, it's finally happening.

A festival that was always an economic development plan

What the Fluff? didn't start as a marshmallow-themed street party for its own sake. It started as a tool. Union Square Main Streets, the nonprofit that still runs the event alongside the Somerville Arts Council, built the festival to draw people into a commercial strip that needed the traffic. The bet was that a weird, hyper-local celebration of an actual Somerville invention would get people to walk the square who otherwise wouldn't, and that some of them would come back.

The invention itself has its own footnote worth knowing if you live here and have never asked. Archibald Query, a Somerville confectioner, sold his marshmallow spread door to door starting around 1917. In 1920 he sold the recipe for $500 to Durkee-Mower, the company that turned it into the Fluff sold in stores today. Durkee-Mower's factory has run out of Lynn ever since, using a formula that hasn't changed, and the company still manufactures somewhere between 5 and 7 million pounds of it a year, with roughly half of that going to customers in New England and upstate New York.

That's a nice piece of trivia to bring up at a cookout. It's also the reason Union Square gets to claim a genuine, singular origin story instead of a generic Main Street identity, which is exactly what the festival's founders were trying to monetize into foot traffic back in 2005.

The evidence is in what's leasing space right now

Twenty-one years is long enough that you can actually check whether the plan worked. This year, for the first time, the answer shows up in real leases rather than attendance counts.

Three things are happening in Union Square right now, each occupying a footprint that used to hold something else:

• First Crush, a natural wine bar and event space, is opening this fall at 259 Washington Street, in the space that used to be Juliet Social Club. Juliet Restaurant is still operating in the same block. Owner Gabrielle Malina ran First Crush as a two-year run of pop-up wine events around Boston before landing a permanent home, and she's noted that the new spot sits just around the corner from Counter Culture, where she hosted her very first event.
• Kima Cafe is targeting a mid-September opening at 75 Bow Street, in the former Fortissimo Coffeehouse space, with a menu built around Korean and other Asian influences.
• Workbar signed a July lease for a 10,000-square-foot coworking facility in the square, expanding a footprint that's been part of Union Square's office mix for years.

None of these are new construction. None of them are filling a hole that's been empty for half a decade, the kind of long dead storefront you see stall out a commercial strip elsewhere in Middlesex County. They're succession. A wine bar is replacing a social club. A café is replacing a coffeehouse. A coworking operator is scaling up an existing use. That's what a commercial district looks like when operators are competing for space rather than landlords struggling to fill it.

Malina put it plainly when she announced the location: "Still in disbelief that we snagged this spot in the heart of Union Square." That's not a landlord's relief at finally getting a signature on a lease. That's an operator's excitement at winning a spot other people wanted too.

What this actually tells you about the square

The easy version of this story is "new businesses come to Union Square every year, cool." That's true and also not the point. The point is that the same commercial corridor the Fluff Festival was built to prop up in 2005 is now generating enough demand that operators are fighting over specific addresses instead of the city having to court them.

You can see the same pattern in who's showing up to the festival itself. Somerville Mayor Jake Wilson has pointed to local businesses, artists, and volunteers as the reason the event has stayed fresh for more than two decades, and this year's commemorative festival design came from local artist Jen Gubicza rather than an outside agency. The event still runs on the same civic infrastructure it always has. What's changed is the commercial fabric around it.

That's worth knowing if you've lived in or near Union Square for a while and have watched the square go through a few different identities. The square that needed a fake holiday to get people to walk down Somerville Avenue in 2005 is not quite the same square that now has a wine bar owner turning down other cities to be here.

What to actually expect this fall

If you're planning your own walk through the square in the next few weeks, here's the practical version. Kima Cafe is aiming for a mid-September opening at 75 Bow Street, so it may already be pouring coffee by the time you read this. First Crush doesn't have a confirmed date yet beyond "this fall" for 259 Washington Street, so keep an eye on its Instagram if you want to be there on day one. Workbar's 10,000-square-foot buildout is a longer runway and likely won't be visible from the sidewalk for a while.

None of that changes what happened Saturday. The festival closed the streets, 100-plus vendors sold fluff-themed everything, and a costumed aardvark took photos with strangers. That's the version of Union Square that shows up on a poster. The version that shows up in a commercial lease is the one that tells you whether the neighborhood's actual bet, the one made 21 years ago, paid off.

It looks like it did.

If you're watching Union Square, Davis Square, or any other corner of Somerville and want a read on what a specific block or building is actually worth right now, that's the kind of question Mike Cohen answers for a living. Reach out anytime you want a straight, local take on the market around you.