To celebrate, here is her first appearance and a major turning point in Supergirl's life!
Action Comics #252 (May, 1959)
Sent to Earth from Argo City (a section of Krypton that had survived the destruction of the planet, for a while) by her father, Zor-El, Kara came to Earth.
That status quo lasted for a while, with Linda getting adopted by the Danvers, but Supergirl was still a secret to the world at large until...
Action Comics #285 (February, 1962)
From here, the world looked a little brighter with Supergirl in it.