The World's Big Sleep Out, organised by Josh Littlejohn and his fellow bosses at Scottish charity organisation Social Bite, will take place in London and New York, as well as Dublin, Ireland, New Delhi, India, Santiago, Chile and Hong Kong.
Executives hope the project will raise $50 million (£38.5 million) to help house those living on the streets, and they officially launched the initiative on Tuesday, October 22, with a one-minute video posted on the programme's social media channels.
The short film opens with Will Smith saying, "Think for a minute about what home means to you. For me home means conversation. It's the place where my family comes together and that's the most beautiful part of home. But there are over 100 million people in the world that don't have a place to call home."
Dame Helen then adds, "On December 7, 2019 we are hoping for 50,000 people to sleep out in the world's largest display of solidarity and support for homeless and displaced people. Major sleep out events will be happening in Trafalgar Square, in Times Square and in 50 cities across the world."
The video concludes with Littlejohn urging viewers to "come and join us... at an official event" or host "your own sleep out".
On the night itself, Smith will join the crowds camping out in New York's Times Square, where he'll read a bedtime story, while Dame Helen will help lull campers to sleep with a tale in London's Trafalgar Square.