You'll hear "Kalimera" all over Greece, from the staff at your hotel to people you see on the street. "Kalimera" is used to mean "good day" or "good morning" and derives from both kali or kalo ("beautiful" or "good"), and mera from imera ("day").
When it comes to traditional greetings in Greece, what you say depends on when you say it. Kalimera is especially for the morning hours while "kalomesimeri" is rarely used but means "good afternoon." Meanwhile, "kalispera" is meant for use in the evenings, and "kalinikta" is meant to say "good night" right before bedtime.
You may combine kalimera (or hear it combined) with "yassas," which is a respectful form of greeting by itself meaning "hello." Yasou is the more casual form, but if you're encountering someone older than you or in a position of authority, use yassas as a formal greeting.
Familiarizing yourself with as many common sayings and phrases as possible before your trip to Greece will help you bridge the culture gap and possibly even make some new Greek friends. To start off a conversation on the right foot, you can use monthly, seasonal, and other time-sensitive greetings to impress the locals.
On the first day of the month, you will sometimes hear the greeting "kalimena" or "kalo mena," meaning "have a happy month" or "happy first of the month." That greeting probably dates from ancient times, when the first day of the month was observed as a mild holiday, somewhat like Sundays are in some places today.
When leaving a group for the evening, you could use one of the "good morning/evening" phrases to express a fond farewell or simply say "antío sas," which means "goodbye." Keep in mind, though, that kalinikta is only really used to say "goodnight" before bed while kalispera can be used throughout the evening to essentially say "see you later."
Benefits of Using the Language Respectfully
When traveling to any foreign country, being respectful of the culture, history, and people is essential, not only to leave a good impression but to ensure you have a better time on your journey. In Greece, a little goes a long way when it comes to using the language.
Like in American etiquette, two good phrases to remember are "parakaló" ("please") and "efkharistó" ("thank you"). Remembering to ask nicely and give thanks when someone's offered you something or provided a service will help you integrate with the locals, and will likely get you better service and treatment.
Additionally, even if you can't understand Greek, many people who live there also speak English, and a number of other European languages. Grecians will appreciate that you've made an effort if you start off by saying "kalimera" ("good morning") or if you end a question in English with "parakaló" ("please").
If you need help, just ask someone if they speak English by saying "milás angliká." Unless the person you meet is downright unfriendly, they'll likely stop and help you out.