To refute the argument, one has to show that at least one of its premises is incorrect. Let us begin with the premise (a).
I want to consider whether it is true that what is worth celebrating is something rare. It seems so at first sight. We celebrated when a man who was originally born in Japan won the Nobel Prize for literature. I myself celebrated when a student of mine passed the entrance examination of a university which he had long wanted to enter the most. I would celebrate if my mother won the lottery the tickets of which she has been in the habit of buying. We celebrate when a happy and rare thing has happened to people around us or when they have achieved something rare and respectable.
But a careful examination reveals that we sometimes celebrate things that are not so rare. We celebrate our weddings. We celebrate a baby is born. We celebrate when young people come of age.
But these counterexamples alone do not prove the invalidity of the premise. It might be that we celebrate what is actually not worth it. So, I have yet to ponder why we celebrate those not uncommon things and find a way to argue that it is still worthwhile to celebrate our birthdays.