How to say hello in Japanese largely depends upon when you say it.
This is very much like different greetings and salutations used in English at different times of day or night.
There are set rules to use "Good Morning!", "Good Afternoon!", "Good Day!", "Good Evening!" and "Good Night!".
In Japanese culture, the rules also differ depending upon whether you are on phone or meeting somebody personally or whether you have only formal relations with the addressee or is he a close friend of yours.
You see, there is nothing strange or new in it.
We too follow the same rules.
We don't get formal with our pals.
We seldom greet them with "Good Morning!" or "Good Evening!" A mere "Hi!" is enough.
Let us see what greetings are to be used when.
Although the most popular and most well known translation for "Hello!" remains "Konnichiwa!" it is not exactly how to say hello in Japanese.
Perhaps it is not the most suitable for it.
Actually, the correct greeting in English to which "Konnichiwa" is closest in sense, is "Good Day!" or "Good Noon!" and I am sure you won't like greeting a person with a "Good Day!" at any time, more so when you actually want to say "Hello!" whose meaning is very close to "Hi!" or "How do you do?" or "What's up?", and neither do Japanese like saying "Konnichiwa" when they want to say "Hello!" So, What to do? The problem still remains how to say hello in Japanese.
Let's do it the way Japanese do.
Use "Ohaiyo Gonzaimasu!" for "Good Morning!" or use "Konnichiwa!" when its noon or use "Konbanwa!" to greet somebody when its afternoon, which is same as "Good Afternoon!" Things differ when you are on phone.
Just say "Moshi, Moshi!" which is actually similar to saying "Hello!" over the phone, because one seldom uses "Good Morning!" or "Good Afternoon!" or "Good Evening!" right after one picks up the phone.
It is invariably the sweet old "Hello!"