Betty White's Assistant Shares 'One of the Last Photos of Her': She Was 'as Happy as Ever'

"She was radiant and beautiful," Betty White's assistant Kiersten Mikelas wrote alongside the picture, which she said was taken on Dec. 20

Betty White is being celebrated by her assistant on what would have been the television legend's 100th birthday.

On Monday, Kiersten Mikelas shared a photo of White — who died on Dec. 31 at the age of 99 — looking "radiant" in a bright green outfit.

The photo showed the Golden Girls alum seated on a floral loveseat, smiling with her hands folded on her lap.

"Hello Everyone! It's Kiersten. Betty's Assistant," she wrote on the star's official Facebook page. "On this special day, I wanted to share this photo of Betty. It was taken on 12/20/21. I believe it's one of the last photos of her."

"She was radiant and beautiful and as happy as ever. Thanks to all of you who are doing kind things today and every day to make the world a better place," she concluded, adding a heart emoji.

In a statement to PEOPLE, Mikelas reflected on White's optimistic mood on the day the photo was taken, saying the two "laughed and joked" together.

"I just want to say that Betty was very excited about filming a special message to her fans for her 100th Birthday. It was important to her because she was so grateful that so many supported her throughout her entire career," she said. "The day we recorded the video and I took the photo, she was excited about being 'glammed up' (especially since COVID had basically eliminated any other opportunities. We laughed and joked and it was a great day. It was extremely important to her that her fans see her and hear from her and I'm so glad that the producers of the tribute were able to include the message."

Countless others have been honoring White on what would have been her centennial birthday through the #BettyWhiteChallenge.

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The challenge picked up steam on social media following the star's death, calling on fans to give $5 to shelters, rescues and other organizations on what would have been her 100th birthday.

"She would be so touched to see and hear that people were honoring her by donating to local groups who work so hard to rescue and rehabilitate animals. I can't think of a better tribute," Mikelas told PEOPLE this week.

According to a rep for GoFundMe, animal-specific fundraisers and donations to animal-related fundraisers have been up in the two weeks since White's death, with other organizations seeing the same change.

"The outpouring of love, the amazing amount of donations being given to all types of animal organizations is such a tribute to how Betty lived her life," her friend and longtime agent Jeff Witjas told PEOPLE. "As Betty would say, 'Thank you from the bottom of my heart.' "

Betty White

White's primary passion outside of her career and her family was for animals. The actress, who rescued several dogs in her lifetime, started working with the Greater Los Angeles Zoo Association in 1966 and joined the board of trustees in 1974.

She also had a long and notable association with the Monterey Bay Aquarium, which is preparing to honor the star by naming their planned giving program in her honor.

In 2006, White was also honored as the City of Los Angeles' "Ambassador to the Animals" for her lifelong work for animal welfare. Eleven years later, in 2017, White received the James Smithson Bicentennial Medal for her "commitment to wildlife and efforts to create a sustainable planet," specifically through her charitable work with zoos and animal rescues.

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