We often think that humans are the only animal which can be happy, or experience emotions, however, this is not true. Animals are also capable of emotions, they are also capable of being happy. In fact, they do experience the same emotions we experience and are often happy.
There is a book called 'Animal wise: The thoughts and emotions of animals' which show how animals experience the same emotions that we do. You can get it on Amazon below.
But, other animals are capable of emotions and happiness too. That has been part of the motivation behind the documentary Blackfish. We are starting to realize that animals, including Orcas. Have emotionally rich lives like we have.
Instead, we treat animals in a Descartes like fashion. Descartes was a philosopher who believed that animals were nothing more than 'automated machines.'
By doing this, we mistreat farm animals. Including; pigs, cows, chickens, ducks and turkeys. We mistreat them because instead of giving them the opportunity to live, and be free, we lock them up in tiny cages and kill them.
We have to realize and accept that animals too have emotions. And that they too experience happiness. Then, we can really call it a 'happiness day'.