Board Thread:General Discussion/@comment-13828134-20140331161323/@comment-4747848-20140427175616

Going by the normal classification of the rarity in Club Penguin, the Cupcake Pin is already rare.

It was released on March 26, in 2010 and became unavailable on April 9, 2010.

So, to answer your question it become 'rare' on April 9, 2014.

Hope this helped! :)