Casimir Pulaski Day is not really a hidden gem. It's a genuine Sufjan Stevens gem that always hits me like a ton of bricks.
I don't really know why, but the lyrics are really sad and the simple guitar-picking is perfectly mellow.
Casimir Pulaski Day is on his album Illinois. If you want to learn more about the Casimir Pulaski Day as it's observed in Illionis, read this article! :-)