It is an example of anthropomorphism, but it’s also potentially part of the human-animal bond, which is why we keep pets in the first place. That bond is important and should be nurtured and if that means your dog (or cat, or fish, or bird, etc) gets a special day once a year, so be it.
The animal probably doesn’t understand the significance of an annual celebration though, the dog probably just enjoys the attention. If you’re both having fun, I don’t see the problem with it.
Lots of people celebrate a ‘gotcha day’ instead of a pet’s birthday though, because they simply don’t know their birthday but do know their adoption day.
The animal has no idea what’s going on, but that doesn’t mean they don’t enjoy the treats and attention. As long as you understand that, go for it.