It's "Bonne journée". I believe it's just a short version of the phrase "Passez une bonne journée" or "Je vous souhaite une bonne journée". Journée is a feminine noun and bonne agrees with the noun.
"Bonjour" means "Hello". You might use this when entering a café and greeting the waiter/waitress, for example.
"Bonne journée" means "Have a good day". You might use this as you're leaving the café, and want to wish the waiter/waitress a good day.
"Bonsoir" and "Bonne soirée" work in the same way as above – one to greet and the other to wish well – but for the evening.