This day was created in 1931 after Henrich Zimmermann first organised the event in Berlin in 1925, moving it to the current day in 1929. It was universally accepted in May 1931 at a congress of the world’s animal protection organisations.
The mission of the day is “to raise the status of animals in order to improve welfare standards around the globe. Building the celebration of World Animal Day unites the animal welfare movement, mobilising it into a global force to make the world a better place for all animals. It’s celebrated in different ways in every country, irrespective of nationality, religion, faith or political ideology. Through increased awareness and education we can create a world where animals are always recognised as sentient beings and full regard is always paid to their welfare.”
The website for the day is here.