You can say "Dzień dobry", what literally means "Good Day", bacause there is just one "Good..." for whole day in Polish language. In English you have "Good morning", what means "have a good morning" and "Good afternoon", what means "have a good afternoon" and in Polish you say just "Dzień dobry", what means "have a good WHOLE day"
I hope you'll get my haywire explanation ;p.
