The Queen of baking and the Duchess of Cambridge began filming months ago, and the documentary will see the pair attending various royal engagements alongside the Duke of Cambridge, Prince William.
Details of the show were revealed earlier this week, and royal fans were treated to a sneak peak of the episode.
While visiting a homeless charity, Passage, Mary asked William about why he is so dedicated to his work.
‘Growing up, both of my parents were hugely charitable. My father set up the Prince’s Trust. He’s involved in so many different organisations,’ he replied.
‘My mother has done her work with homelessness and Aids and other charities and I think I’ve grown up in a household which has been very much a case of, “Well, we’re very lucky – you must give back.”‘
William also decided to open up about his grandmother, the Queen, and the struggles she faced as a young monarch in ‘a man’s world’.
He said: ‘My grandmother becoming Queen at a very young age, in the days when it was a man’s world, it was very difficult for her to make a difference. And she’s done it. In her own very unique, distinct way, and you know, my grandmother and my grandfather together are the epitome of public service.’
He continued: ‘I personally get a lot out of helping people.
‘I find that you hear and you understand and you learn so much more by giving a bit of your time, a bit of your day, to just be around.’
A Berry Royal Christmas airs on BBC One, Monday 16 December at 8.30pm.