"Good morning and good night!"
Translation:Καλημέρα και καληνύχτα!
This might help you remember for that one specific case:
Καλημέρα is composed of καλή (good) + μέρα (morning), and it's a greeting (or a wish. Whatever suits you.) Most wishes in Greek are usually composed of two words, an adjective and a noun, like in english.
The adjective and the noun have to match in gender in order for it not to sound or look awkward.
Καλός (m.), καλή (fem.), καλό (n.)
In this case, both the adjective and the noun are of feminine gender (καλή + μέρα) , so it matches. Since there is no such thing as καλί, καλιμέρα, might not sound, but would seem awkward, and wrong. It's not gramatically correct.
I hope I helped you. ^.^