There was a time, though, when he had a bright idea: On those off nights, he'd deploy an all-female waitstaff, three of them ballet dancers.
"We would be packed," he mused. "There's something about when a ballet dancer walks down the floor . . . ."
Who knows what the most critical ingredients were - good instincts (a $10-over-cost wine program), an uncommonly loyal staff (the head waiter has logged 32 years), location (21st Street north of Spruce was a restaurant desert), an eye for beauty (Lilley was and is a commercial photographer), a taste for mischief (servers promenaded in the cloak room in the guests' fur wraps).
But whatever they were, this year, Friday Saturday Sunday crossed into legendary territory.
It has turned 40 now, the oldest full-service restaurant still cooking in Center City, and the last survivor of a Philadelphia restaurant renaissance that in the early '70s lit a fire under a sad, stale, moribund dining scene, upending for good the way we eat out.
In the current fevered charm, with restaurants popping up like chanterelles, only to wilt just as quickly, and when staying solvent for 10, or rarely, 20 years, is miraculous, the house that Lilley built is not simply an outlier, but darn near freakish: 40 years!
Buyers have come sniffing. But Lilley, 70 now, has no plans to sell: "I want to keep it going forever."
He and his wife Jamie have two sons, 19 and 23. It is hard to predict, he says, if they'll ever step in. Weaver Lilley himself certainly never had a plan.
It was a different time, before the fall of Saigon, before the rise of Yelp. There were a handful of restaurants in the city, "still opening cans in the kitchen," reflects Lilley, "or throwing a steak on the grill. We were cooking!"
Something was stirring. Young people were rebelling against the war in Vietnam, against their parents' choices, reaching for something genuine, fresh, "from scratch," is Lilley's phrase.
But not until Jay Gubin, who would later start the Restaurant School, dared Lilley and another pal - in a smoke-filled room - to pony up $2,000 apiece to open a restaurant, did the ball get rolling.
With four more partners - including Annie Perrier, chef Georges Perrier's wife at the time - the kitty eventually hit $14,000, enough to buy six used apartment refrigerators, which were lined up in the basement, and get construction underway.
Perrier himself had recently opened his game-changing Le Bec-Fin. And a week or so before Friday Saturday Sunday's debut, Steve Poses' seminal storefront Frog (now closed) had opened, with the FriSatSun crew lending a hand.
In West Philadelphia, La Terrasse (now closed) was changing the temperature, its blackboard menu hanging from a leafy tree that loomed in the dining room, the inspiration for Lilley's own blackboards.
Lilley designed the restaurant space himself, a comfortable, mildly funky room, low-lit and moody, the upstairs Tank Bar featuring a 135-gallon fish tank, the fish lazing about under a billowing tented ceiling. It's a vaguely hippie vibe; "like living in your pajamas," he said.
He was in it for a lark, he says now. Back then, he was so exhausted he came down with a virus and couldn't make opening night.
Most of the partners had college degrees and day jobs. The dishwasher had a Ph.D. They'd take turns running the place on the weekends, open at first only on Friday, Saturday and Sunday.
Two years later, it was clear it wasn't going to work as a hobby. Lilley was disheartened; it looked like the gig was up. On a drive to north of New Hope on the Delaware, a friend told him to man up: "She said, 'Why don't you buy it?' "
He did just that, buying out his partners for $5,000 apiece. Then he bought the building, once an infamous coffeehouse called the Gilded Cage.
And, day by day, he opened the place up all week long.
When I'd first arrived for my recent visit, Weaver Lilley - his vintage tie tucked in his work shirt - was running on fumes.
He'd been up and down ladders for the last three days - in fact, until three hours before - employing a stapler and Velcro to re-drape the tented ceiling in the upstairs Tank Bar.
We raised a Manhattan to the new ceiling, and admired the new pin-striped fabric.
At the next table, a dapper local named David Burns was polishing off a lamb shank.
How long had he been coming here?
Since 1973, he said.
How many times? A dozen?
"No," he said, "more like 30 or more . . . ."
So it goes at Friday Saturday Sunday, the work still in progress.
It was cutting-edge in its day, reeling in the dating crowd. It is resolutely untrendy now, an advantage, perhaps, more than a disability, in a city of hundreds of restaurants today, more than 75 in the Rittenhouse neighborhood alone.
It is resolutely, as well, a favorite neighborhood spot. And while that may be one of the secrets to its success, Lilley calls the neighborhood restaurant a "threatened species."
Each new flavor of the month - offering Brooklyn-style ribs, a British pub's open hearth, a craft-beer garden - siphons off a younger crowd and beckons to curious old customers: "People who'd come in every two weeks now maybe come in once a month."
So FriSatSun is trying harder. It's looking for better crabmeat for its iconic crab cake.
The new head chef Brendan McLaughlin is reengineering the bar-menu burger.
Lilley thinks the filet tips they've been using are too lean. He wants it juicier, the fat dripping out of the bun.
And this month - in typically wry tribute - servers are getting a new T-shirt: "40," it says. "Not me the restaurant."
Contact Rick Nichols at firstname.lastname@example.org.