The milestone birthday bash, which will take place at the Saatchi Gallery on the Kings Road, is set to reunite Maurice and Charles Saatchi with their former agency for the first time in six years.
The two brothers founded Saatchi and Saatchi in 1970 but both left in 1994 after internal tensions led to Maurice being voted off the board.
The fall-out resulted in Mars ripping its £250m advertising business out of the group's Bates agency and into rival AMV. BBDO.
Further turmoil followed as the agency also lost the Dixons, Mirror Group Newspapers, British Airways and Gallaher accounts to the brothers' start-up, M&C Saatchi, which launched the following year.
The Saatchi brothers also poached a team of top executives, such as Moray MacLennan, who is now M&C Saatchi’s worldwide chief executive.
In 2007, Saatchi and Saatchi defected from its long affiliation with the Conservative Party to pick up the Labour Party account, leaving M&C to take the Conservative account in March this year.
The acrimony between the two agencies may have abated over the years but the rivalry remains. Earlier this year, the two agencies worked for warring political parties in the General Election.
M&C Saatchi joined forces with Euro RSCG to create campaigns for The Conservative Party while Saatchi & Saatchi were behind Labour’s efforts, including the slogan "A future fair for all".
Moray MacLennan, worldwide chief executive of M&C Saatchi, confirmed the brothers are able to attend, and said: "We remain rivals, like we are with other advertising agencies. However we are friends and rivals now.
"The party is a celebration of the Saatchi and Saatchi brand. It’s a thank you to clients and people that have worked together over the past 40 years celebrating the Saatchi phenomena. It’s also to introduce fun into the gloom; it will be a joyous occasion."