Last year, I put an elf hat on AJ during the Winter holiday season…but I didn't do it to "humanize" him. Rather, he was performing a service to humans by making them smile. This year, I was planning to get a Santa hat with whiskers and small saddle bags for him and take candy-canes around the terminal. In my opinion, doing this is no different than taking a service dog into a hospital to cheer up the patients and assist with the healing process. As has been shown in study after study, people tend to be more depressed during Winter, especially between Thanksgiving and New Year's Day. Adding a little cheer to their holiday season is part of a dog's job. AJ doesn't seem to mind...he likes the attention. However, he walks himself and still understands he is the dog.
I really see no issue with this, but only if he is comfortable with it. I don't believe I am "humanizing" him by doing this. As far as special things around holidays, they know something is going on. Why not make it fun for them too? (My idea of that is, yes, he will get a toy as a gift and he will get something special in his dinner dish, but that's as far as it goes. He usually has more fun with the wrapping paper anyway.)
Arranging "play dates" for dogs, I think, is a good idea. The involved owners know where and when to meet and there is more control over which dogs are involved. I believe this is better than just heading off to the dog park and hoping there are no bullies there. (As discussed in a different thread, some people do not recognize aggression in their dog, but rather say, "Look, he's being so friendly and playing." when their animal is actually picking a fight.) Perhaps calling it a "play date" is the objectionable issue.
A birthday party for a dog? Forget it. Not in this truck. It's just another day to a dog and they don't understand what's going on.
As far as a Halloween costume, unless the dog is part of the child's repertoire, there is no need for it and most dogs don't appreciate it anyway. What are they going to trick-or-treat for? I never kept dog cookies for canine visitors at Halloween. Come to think of it, nowadays, it's not a bad idea to send the dog out with the kids trick-or-treating. I'm just not so sure about the costume.