Kindness is always in season. 

Blake Shelton, Jack Black and Millie Bobby Brown were just a few of the many stars to share the importance of spreading kindness around like wildfire as they posted uplifting messages across social media on World Kindness Day.

The country crooner encouraged his fans to get into the spirit while tweeting, "Let’s spread some kindness y’all!!! Join the Kindness Challenge with me by following #OnOurSleeves."

Black, who also headlines as the lead singer of "Tenacious D," held onto a puzzle-piece shaped heart as part of a campaign with Autism Speaks.

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Jack Black and Blake Shelton smile while posing for photos

World Kindness Day is marked on Nov. 13, with celebrities participating online and in various campaigns around the globe. (Getty Images)

Brown, who launched into super stardom on "Stranger Things," posted a video on her new beauty Instagram page for her millions of followers.

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"Happy World Kindness Day," Brown said in a quick clip explaining "how important it is to be kind to each other, to yourself and to the world." 

She added, "The act of kindness can make all the difference in our lives. Compliment that person on the street, check in with an old friend, volunteer or even smile at someone."

Colin Wayne, a decorated US Army Veteran and CEO of Redline Steel, created the puzzle piece artwork to benefit the nonprofit. 

Wayne said, "This World Kindness Day, I am proud to partner with Autism Speaks in an effort to advocate for the autism community. Autism affects 1 in 44 children in the USA, and while it can vary in severity, all individuals with autism deserve to be treated with kindness. We believe that by working together, we can provide affected families with the tools and resources they need to help individuals with autism thrive in a kinder world."

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Country singer Cutter Elliot said the "Be Kind" design will help promote kindness toward the autism community and beyond. 

"You've got to try a little kindness, show a little kindness, shine your light for everyone to see, and if you try a little kindness then you'll overlook the blindness of the narrow minded people on their narrow minded street," Cutter said. 

"When you are kind and you treat people kind, you get treated in kind. I'm proud to show my support for Autism Speaks and Redline Steel, two organizations that have come together to spread kindness."

Kindness wasn't just relegated for the real world. "Sesame Street" star Elmo tweeted, "There are lots of ways to show kindness! How are you showing kindness today?"

Kermit the Frog also offered some words of wisdom, "The best thing you can be isn’t rich or famous, the best thing you can be is kind. Let’s all be kind to each other, today and every day. #WorldKindnessDay. 

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