The establishment of his company, to be called Passionbrand, comes three months after the end of his contract at JWT, where he had been working since the summer of 2000.
His partner in the venture is Helen Edwards, a lecturer on brand management at the London Business School and a former consultant at Butterfield 8, the consultancy established by Leslie Butterfield.
Day and Edwards have co-written a book, Creating Passionbrands, which will be published in June. The book looks at a number of brands, including Google, Dyson and the Co-operative Bank, which have been particularly successful at communication and examining the reasons for what makes them special.
Day, one of the founders of Partners BDDH, said the consultancy was the result of his agency experiences, which made him realise that the most successful brands were those that connected with consumers on a deeply emotional level.
"A lot of the strategic advice given to clients is like foreplay without the orgasm," he said. "At that stage, clients have to work out how to implement that advice across a wide range of media."
Passionbrand will not produce ads but Day explained that it would aim to stimulate creatives into delivering their best work on a brand.
A one-time senior creative at Abbott Mead Vickers BBDO, Day left the agency in 1986 to become the creative director of the then Ted Bates agency.
He helped found BDDH in 1987 with Butterfield, Mick Devito and Michael Hockney, now the D&AD chief executive.