Mother is a founding partner, along with Harriet Quick, the fashion features editor of Vogue magazine. Erik Torstensson and Jens Grede, both from Wink Creative, are the other two founders.
Mother owns a significant minority stake in the venture, which will offer brand development, strategic consultancy and television advertising. It will operate separately from the main ad agency, in the same way that Mother operates its joint ventures with Naked and the digital creative agency Poke.
Quick owns a stake in Saturday and will offer creative and strategic advice but will not be based at the agency on a day-to-day basis. She maintains her role at Vogue. It will employ around ten staff in its first year of operation.
Grede will be the managing director of the venture, while Torstensson takes the creative director's title.
"We came to Mother because it has such a strong creative viewpoint and we didn't want to be part of a large network," Grede said. "There is a big gap in the market as clients are not always getting what they pay for at big global brand consultancies."
Grede said that Saturday is already working with a number of as yet undisclosed clients.
Mark Waites, a founder of Mother, said: "Working with Saturday continues our desire to work with the smartest people we can find. They're design entrepreneurs and, like Mother, they understand the power of working direct to clients."