The most common type of persons who make this mistake are ones who want to be formal, but don’t know how to do it right, so they do it wrong, and end up being pretentious instead of formal.
Don’t write “as per”!!! It’s wrong! It’s a pretentiousism. Harrumpf.
Here’s the comic. Comics are a good place for this poor grammar, and it fits rather well into this guy’s persona.
He had so many ways to say this correctly: according to, in line with, as we traditionally do, as usual; even just end the sentence after “Thanksgiving.”