On a request from some friends I was asked to explain the greetings Greeks use during the day. I take this opportunity to explain for those who are not familiar.
ΚΑΛΗΜΕΡΑ / Καλημέρα (Kalimera) = good morning. This is used from early morning till midday. On the other hand if you use the two words separately Kali (sou /sas) mera = have a good day. This can be used as a parting greeting again till midday. Alternatively also in parting Να έχεις / έχετε μία καλή μέρα (Na eheis / ehete mia kali mera) = May you have a good day. The remainder of it. [eheis (singular), ehete (plural)]. A very colloquial but friendly and joking greeting for the morning is Kalimeroudia. To be used only among people you know well. Others will not be offended but they will be surprised of how good Greek you know.
From 12.00 onwards it is Καλό μεσημέρι (Kalo mesimeri) which is used up until 15.00-16.00 wishing good noon, but in Greek it also conveys the wish to have a nice lunch and a nice siesta. Especially as a parting greeting.
From about 16-17.00 it is customary to use Καλό απόγευμα (Kalo apogevma) = Good afternoon. This can be used till about 18.00, especially during summer that is still daylight on. [Από-γεύμα = after lunch]
From before dusk ΚΑΛΗΣΠΕΡΑ / Καλησπέρα (Kalispera) = Good evening, is used and which can be used till late. Sometimes it is also used as late as 24.00.On this occasion the greeting of Kalisperoudia can also be used like Kalimeroudia, especially amongst friends. When joining someone or a group especially just about before sundown it is best to use Kalispera. It is never Kalinichta or Kalinychta. I have seen this many times used wrongly by people. Although your good intention is apparent the greeting is wrong.
Upon leaving someone we use Καλό βράδυ = Kalo vradi if it is early in the evening and certainly before midnight. Καληνύχτα (Kalinichta or Kalinychta) is always used when departing from a company or a friend and the meaning is to have a nice and uneventful rest of the time before you go to sleep. The additional wishes may be used after kalinichta: Καλόν ύπνο (Kalon Ypno) = Sleep well and Καλό ξημέρωμα (Kalo ksimeroma) = May you have a happy awakening in the morning. Also, Όνειρα γλυκά (Onira glyka) = Sweet dreams.
I hope these explanations can cover the subject and even if you make a mistake we know that you mean well and it is taken as such. Have a good day my friends and I hope I have enlighten you in some simple way.