I do not celebrate my dogs birthday as I understand that he is a dog and only a dog. He might be part of my family, but he isn't human and understand the cultural ritual that is a birthday. If he had one the only thing he would be interested in is the cake, which is bad for him as dogs are not suppose to be eating sugar. They can potentially die from it.
I know there are people out there that do celebrate their dogs birthday. As long as you aren't hurting anyone or infringing another persons liberty or freedom, then go for it. Just don't expect a doggy gift from me. I don't see how a dog can understand a birthday let alone gifts.