Zelda Williams honored her beloved father, Robin Williams, on what would have been the late star's 65th birthday

Credit: Source: Zelda Williams/Instagram

In a heartbreaking message and photo, Zelda Williams is honoring her beloved late father, Robin Williams, on what would have been his 65th birthday.

“Still not really sure what to do on days like these … ” Zelda, 26, wrote in a lengthy Instagram post shared Thursday alongside a vintage black-and-white photo of Williams goofing off with her younger self alongside brothers Zach and Cody.

The Oscar winner died by suicide in August of 2014 at his home in Tiburon, California. His widow, Susan Williams, later told PEOPLE he was struggling with Lewy Body Dementia, a neurodegenerative disease that causes fluctuations in mental status, hallucinations and impairment of motor function.

RELATED: ‘It Was Not Depression That Killed Robin,’ Says Widow Susan Williams

In Thursday’s message, Zelda explained that she has found one way to honor her father’s memory: donating to causes close to his heart.

“I know I can’t give you a present anymore, but I guess that means I’ll just have to keep giving them in your name instead,” she wrote.

“This year, I tried to help three causes you cared about in one; rescue dogs, people suffering from disabilities, and our nation’s wounded veterans, so I donated to freedomservicedogs.org.”

The Dead of Summer star went on to explain that the charity rescues dogs “from shelters and train them so that they can be paired with someone in dire need of their help and companionship. Thought you’d get a kick out of furry, four legged friends helping change the world, one warrior in need at a time.”

Zelda made her grief clear as she concluded her message: “Happy birthday Poppo. Shasha, Zakky, Codeman and I all love you and miss you like crazy. Xo”