At Friday Saturday Sunday, secret cocktails are based on ancient math

Paul MacDonald is not a barman who conjures concoctions by simple chance or common ratios. The Friday Saturday Sunday bartender is known for crafting cocktails to the logic of a 13th-century mathematical formula. In the eponymous Fibonacci sequence — named after an Italian numbers whiz also known as “Leonardo of Pisa” — each number is the sum of the two preceding numbers, beginning with 0 and 1, then another 1, 2, 3, 5, 8, etc. For almost a millennia, the simple series has been seen as a numbering system to the cosmos.

MacDonald may be the first to apply the mathematical solution to booze. His cocktail creations — like his signature: Assassin’s Handbook, a blend of cognac, Jamaican rum, averna, amaro, and mulled wine, with a dash of habanero tincture — rely on ratios based on the famous sequence. The drinks are praised for their understated complexity, balance, and slowly unfolding flavors.

“I like a drink that underpromises and overdelivers,” MacDonald said, stirring up a cocktail.

“He’s a complete wizard,” said Hanna Williams, who runs the elegant Rittenhouse Square rowhouse eatery with her husband, chef Chad Williams. Earlier this month, Friday Saturday Sunday won the prestigious James Beard Award for outstanding restaurant.

But even before the nation took notice, the Williamses’ tasting menu had been lauded as one of the singular dining experiences in Philadelphia. The Williamses credited MacDonald’s cocktail creativity and welcoming vibe as critical to the restaurant’s success.

“He has the ability to make something special for everyone,” Hanna Williams said, during a recent Friday evening. The bar hummed even more than usual in the wake of the award.

Added Chad Williams, modestly: “Down here is really the soul of the place. When people think of Friday Saturday Sunday, they think of Paul at the bar.”

MacDonald’s lauded drink menu contains eight — and only eight — offerings. (“The program is intended to be a conversation starter with guests,” he explained.) But at his constant recall are dozens of other mathematically divined, off-menu drinks. Ones that he still may be developing — or old favorites no longer on the abbreviated menu.

“It’s more about what’s not on the menu than what’s on the menu,” MacDonald said.

MacDonald, 35, who is married with two children, lives in East Falls after years in South Philly. He entered bartending after graduating from Moravian University with a music degree (a talented amateur photographer, he documents his creations on Instagram.) Before joining the Williamses at Friday Saturday Sunday, he fine-tuned his craft at another Rittenhouse drinkery, a.bar. He adopted his mathematical method six years ago, while developing a drink with five fortified wines. He realized his recipe ratio — ¼ ounce, ¼ ounce, ½ ounce, ¾ ounce, 1 ¼ ounce — aligned with the Fibonacci sequence.

“I learned about it in high school,” he said.

He’s not claiming a miracle ratio for the perfectly balanced cocktail. But imposing the artificial constraints of the formula can spark creativity and new ideas, he said.

“It forces you to think about the relationships between flavors in a different way,” he said.

In the craft cocktail-dominated bar scene, even MacDonald’s most loyal regulars say his delicious drinks are just the starting point.

“Having great drinks are the table stakes now ― and he exceeds that,” said Gene Gualtieri, settling into his regular stool. A research engineer with a Ph.D in nuclear physics, Gualtieri appreciates his favorite bartender’s exacting precision.

He was one of four regulars who traveled to Chicago to cheer on Friday Saturday Sunday at the James Beard Restaurant and Chef Awards, held June 5 at the Lyric Opera House.

“I didn’t want to live in a world where they won and I wasn’t there,” he said, sipping MacDonald’s sherry martini, an off-menu selection made with fino sherry instead of vermouth. “I yelled really loudly when they won. I think I scared everybody around me.”

That’s the sort of bartender’s bond MacDonald aims to build over his cocktails.

“At its heart, it’s still a neighborhood bar in Rittenhouse Square,” he said.