Something along the lines of 'hero of the occasion', but specifically for birthday? If there isn't, how would you otherwise say that?
('the subject of birthday party', 'the hero of this birthday party', 'birthday's hero' all sound stupid somehow).
My first reaction was that the birthday boy or girl is the "Fetted" person, which upon research appears to not be a (current) dictionary word!
For what it’s word Here are some usages of it from culture, via https://www.wordnik.com/words/fetted:
In hindsight, asked Marr "is there anything different you might have done over the past decade, anything you did wrong ... maybe became too enthrawled by the magic of these bankers you knighted, fetted, appointed to high office?"
Sorry Seems to Be the Hardest Word
Living in an age when women as writers and playwrights were not as evident or plentiful as they are today, Hellman felt she could take liberties with the truth knowing full well how Hemingway and other fetted and celebrated male writers were also bound to fudge the truth, or embellish.
The scandalous Lillian Hellman
It seems to be very fringe but a fun and useful word, despite the similarity to "Fetid".