In all of the most popular languages, we have different greetings for different parts of the day. In some languages the day is divided to many pieces, in some just 2-3. Today we are going to have a look at the Turkish ones through English.
Günaydın: İs direct equivalent to good morning in English. However when we break down the word we can come up with the meanings such as "the day is bright", "bright days", "the sun is shining" ...
There are some less used alternatives to this word such as "iyi sabahlar" (good mornings) or "hayırlı sabahlar" (blessed mornings) or even "sabah şerifleriniz hayrolsun" (This one is the wish version of the previous one)
Please remember, the most common one is "günaydın".
İyi günler: This is the tricky one. Because you probably were expecting an equivalent of "good afternoon". Well, yes and no. First of all, we don't practically use the concept of "afternoon". The closest concept to afternoon in Turkish is "akşam üzeri" (evening-ish) we can roughly decribe it as later afternoon to early evening with not clear starting time but ends before the sunset starts.
This phrase directly means "good days". It can be used instead of good afternoon and have a nice day practically.
İyi akşamlar: Nothing fancy here. This one means good evening.
İyi geceler: This one is quite direct too. Good night. Commonly used with "Tatlı rüyalar" (sweet dreams).
This video will help you for your pronunciation.