Just to be clear, though, it's a day when you can wear your best (or most hideous) Christmas jumper to work, school, college (anywhere you like) and not be mercilessly ribbed for it.
Okay, that’s a lie. There’s no doubt you’ll be subjected to some mickey-taking by pals and work colleagues, but don't be a bah-humbug... it's all for a good cause.
People taking part in Christmas Jumper Day are asked to donate Â£2, or Â£1 (if you're at school) to Save the Children.
Money raised goes to the charity's on-going projects to alleviate child poverty in Britain and across the globe.
The charity says its aim is to make "the world better with a sweater" - and to be fair, that's a pretty snappy slogan.
A spokesperson for Save The Children says: “You’ll be saving children’s lives, and helping the charity do whatever it takes to make sure the world’s children get the chance to have a brighter future.
“We would love your support – please do get involved, stick on a festive knit and get friends, family and your whole office involved.
“If you want to take a selfie in your jumper we’d love it if you shared these pictures on social media to help raise awareness for the charity.”
This year's Christmas Jumper Day has been backed by a whole host of celebrities, including Fearne Cotton and Myleene Klass - and you can be sure Twitter will be awash with even more A-listers donning their woolly pullies for a good cause.
“There is nothing I love more than dressing my whole family up in Christmas jumpers," says Fearne. "I really don't shy away from over the top novelty designs!
“I love Christmas Jumper Day because it’s so simple to get involved and help raise money for those less fortunate all over the world.”
Myleene loves it, too. "Save the Children's Christmas Jumper Day is always a big part of Christmas fun in my household," she says.
"My girls and I have a day in our diary to Christmas jumper shop. It gets pretty hectic as we scoot through the rails, each searching out the silliest knit!
"I've seen first-hand in countries like Bangladesh the suffering some children go through. By wearing your jumper and donating Â£2 you will help change lives for the better.”
If you don't have a Christmas jumper but want to get involved, there's no excuses. Plenty of retailers stock festive jumpers these days. Alternatively, you can make your own, and the money you save could be donated to Save The Children.
Save the Children suggest making your own jumper by customising a regular sweater with sequins, tinsel or pom poms.
Once you’ve gathered your friends and colleagues together, share your Christmas Jumper selfies using the official #christmasjumperday hashtag.
Sign up and donate Â£2 to Save the Children (Â£1 if you’re at school) at christmasjumperday.org