The one-off instalment, titled 'Mistletoe and Rum', will be available to watch immediately after this Saturday's (December 15) regular edition of the BBC One medical drama.
'Mistletoe and Rum' sees the Emergency Department's nurses on a Christmas night out and also explores the continued chemistry between Fletch and Tess.
With the festive celebrations taking place at a pub, Fletch and Tess spot an opportunity and step outside for some air. However, they are soon distracted when a homeless man named Tyron is sent into a hypoglycaemic coma, leaving the nurses battling to keep him alive.
Behind-the-scenes footage from the making of the episode will be available today (December 12) on the official Casualty website and Facebook page.
Casualty's executive producer Jonathan Young commented: "We wanted to take a closer look at a storyline that is set to escalate in the New Year between two of our most popular characters, Fletch and Tess.
"The Red Button episodes provide an ideal opportunity to do just that and to give something extra to our fans. And with the fantastic behind-the-scenes footage of Sunetra directing, we can't wait to see the reaction!"
Sarker added: "Moving from in front of the camera to behind presents so many different challenges. What I found most refreshing was the respect and trust that everyone gave me as a director and the chance to collaborate with each department in detail. The cast gave me top performances and I had total crew support.
"This is a slice of the nurses' Christmas night out, focusing on Fletch and shows the nursing team off duty down the pub. It's a treat for all the fans that follow the show. I was thrilled to be able to make it."
Casualty viewers can see the episode via the Red Button from around 10pm on Saturday, December 15. It will also be available on the BBC iPlayer.