Here's what to expect from the Call the Midwife Christmas special as Poplar becomes a "winter wonderland"

"We were given an incredible historical gift because Boxing Day 1962 was the first day of the Big Freeze, when snow fell and it was going to be on the ground for – little did anybody know – but three months," Call the Midwife creator Heidi Thomas says. "It was just the perfect setting for Christmas because obviously there's loads of snow – but it's also about people being challenged and having to join forces to survive adversity."

Here's what you need to know about the Call the Midwife Christmas special:

Call the Midwife tells the story of the Big Freeze of 1962-3

Quite literally, this year's Christmas Day episode is miles away from last year's festive special which took us (and the midwives) to a struggling hospital in baking hot South Africa. Instead we have snowmen and christmas trees and stockings; hot tea and musical chairs; boy scouts building igloos; midwives checking the festive TV schedules in Radio Times magazine (yes, really).

It's about as Christmassy as a Christmas episode can get.

"The Christmas episode is just a winter wonderland," says Stephen McGann, who plays Dr Turner. Having actually been born during the blizzard himself, the actor adds: "This Christmas was genuinely freezing. It was famously, famously cold."

Two feet of snow piled up in London during the Big Freeze as the capital ground to a halt. Mothers with newborns were hardly able to go outside in the bitter cold, millions of milk bottles went missing, and Londoners were forced to get their water from standpipes in the street as pipes froze.

In Call the Midwife, the messy business of life, death and childbirth continues whatever the weather, but conditions are tough as the midwives of Nonnatus House battle through the snow to do their duty.

EastEnders legend Anita Dobson guest stars – and there's a lot of depth to her story

Anita Dobson looks extremely frail as elderly woman Mabel Tillerson in the Christmas special. She first appears on the midwives' radar when Dr Turner (Stephen McGann) sends Sister Julienne (Jenny Agutter) on a mission to alert the family of a patient who has unexpectedly died – but not everything is as it seems at first glance...

McGann explains: "What seems like a sad but quite simple case turns out to be something a heck of a lot more complex for the individual, particularly for the individual's family."

"It was wonderful having Anita actually. Really fantastic," Thomas says. "And she was so 'on it'. She came out our read-through looking incredibly glamorous, she's a very beautiful woman, so as ever with a lot of our actors – such as Jenny [Agutter] or indeed glamorous Judy [Parfitt], who play the nuns – it's the absolute lack of vanity a really good actor will bring to a part.

"She's very stripped back and honestly in real life Anita is beautiful, and I just love it when actors are prepared to bring you literally their bare-faced naked selves and invest in a character that we've all created together. I think she's wonderful in the Christmas special."

Trixie and her boyfriend are going skiing – but will they get off the ground?

Nurse Trixie Franklin (Helen George) is preparing for a glamorous skiing holiday with her dentist boyfriend Christopher (Jack Hawkins). They have a plane to catch and Trixie is all about the apres-ski, packing a suitcase full of fancy furs.

Will heavy snowfall disrupt their holiday or will they make it to the slopes?

Barbara and Tom have to make a life-changing decision

Nurse Barbara Gilbert (Charlotte Ritchie) tied the knot with the very earnest Reverend Tom Hereward (Jack Ashton) during the last series, and now they're having their first Christmas together as newlyweds!

But in the middle of all the festivities, Tom gets some news and has to make a big decision...

Nurse Valerie Dyer takes centre stage

Nurse Valerie Dyer (Jennifer Kirby) is enjoying her first Christmas at Nonnatus House – and Heidi Thomas has given her an emotional storyline which will definitely tug at your heartstrings.

"I go and help out a young mother, and the weather conditions play a big part in that, and things evolve," Kirby teases.

We'll find out more on Christmas Day...

The Call the Midwife Christmas special will air on 25th December at 7:40pm on BBC1