The 26-year-old actress posted a throwback photo and heartfelt message to Instagram on what would have been Williams' 65th birthday.
"Still not really sure what to do on days like today... 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," Zelda wrote.
The Dead of Summer actress donated to Freedom Service Dogs of America in her dad's memory. The non-profit organization rescues dogs and trains them to assist people with various disabilities.
"Happy birthday Poppo. Shasha, Zakky, Codeman and I all love you and miss you like crazy," she concluded.
Williams died by suicide at age 63 in August 2014. His wife, Susan Schneider, later revealed he was struggling with Lewy body dementia, a neurodegenerative disease that causes rapid cognitive decline and impairment of motor function.
"There's no point in questioning it," Zelda said of her dad's death on the Today show in 2015. "It happened and you have to continue to move and you have to continue to live and manage."
Still not really sure what to do on days like today… 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. 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. They rescue pups 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. Happy birthday Poppo. Shasha, Zakky, Codeman and I all love you and miss you like crazy. Xo
Williams shared Zelda and son Cody with ex-wife Marsha Garces and was also dad to son Zach with first wife Valerie Velardi. Zelda plays Drew Reeves on Dead of Summer, which airs Tuesdays at 9 p.m. ET on Freeform.