The name speaks for itself.  However, it's origin is a bit sketchy. Zoos I've written about in 2024.

This day is for bird-watchers, and is the last day of the US Christmas Bird Count. Turkey Vulture

This is a day to become more informed about penguins and their current issues.  For example the Galapagos penguin is the only penguin that sometimes swims into the northern hemisphere, it is also an endangered species. King Penguins second only to Emperor Penguins in size

This is the day to appreciate squirrels, for which there are 122 species of "tree" squirrels. Prevost's Squirrel    

This day is to learn and protect zebras.  There are three species of zebra, the plains zebra (doing well), the mountain zebra (vulnerable/threatened), and Grevy's zebra (endangered).

This day is to raise awareness of the endangered great ape: Bonobo.  Though they look very similar to a chimpanzee they are a completely separate species.  Also, whereas chimpanzees are known for their aggression the bonobo is known for its much more passivity.

This day is for raising awareness of the pangolin, for which there are 8 species (4 in Asia, 4 in Africa).  Pangolins are the most illegally trafficked animals in the world. Black-Bellied Pangolin

World hippopotamus day is to celebrate this unique mammals of Africa.

World Whale Day aims to raise awareness of these magnificent creatures, their beauty as well as their ecological value to the planet. \ Fin Whale, my favorite whale.

This day is for celebrating the polar bear the world's largest bear and land carnivore.  They are also the unofficial climate change mascot. Polar Bear, Kansas City Zoo