"bom dia" would literally mean "good day". However it is used like English "good morning".
"boa tarde" is used after lunch till it gets dark.
"boa noite" is used to convey "goodnight" and is generally used after the evening when it is dark.
"bom dia", "boa tarde" and "boa noite" are also used to convey "good bye" at appropriate time of the day.
"olá" is a casual way of saying "hello" in Portuguese.
In Brazil often "oi" is used to say "hello".
"adeus" is a friendly way to say "goodbye" anytime.