The tradition began in 1964 -- when the down-trodden donkey character was already 38 years old -- when several University of Texas students gathered at the park to party in honor of the "Winnie the Pooh" character, the Austin American-Statesman reported Saturday.
That handful of friends hanging out in the park has swelled to thousands of people gathering for a fundraiser for the Friends of the Forest Foundation Saturday.
Marv Howse, 63, who has attended the event for 30 years, says he's seen more and more people attend.
"People like having a good time, and this is a good time," he said.
Non-profit organizations sell food and drink, as people on unicycles play football, and paint-covered and costumed adults play drums at the event, the newspaper said.
"I hope it goes on forever. What could be more fun?" said Karen Phillips, who has attended the party for nearly 20 years. "People here are laid back, open, accepting, welcoming, ready for anything. There's no pretense here."