"Good afternoon!"

Translation: Dobré odpoledne!

[lhgates]

Why is it sometimes dobry and sometimes dobre?

[JanLyko]

Gender. "Den" is masculine (-ý) and "odpoledne" is neuter (-é). See https://www.duolingo.com/skill/cs/Masculine

[Esus111138]

Btw. this phrase is used rarely by native speakers...

[VladaFu]

Maybe not be so common in speech but the written corpus has a decent amount. http://syd.korpus.cz/ClKCXa3i.syn