Buenos días -> until 12:00pm
Buenas tardes -> from 12:01pm to last sun light
Buenas noches -> after sunlight is gone
Buen día differ if used like -> Que tenga un buen día = Have a nice day || Otherwise it can be interpreted as Good morning
Variables such as Buenas are used in some countries | Buenas = Howdy (no time frame).
For those getting doubts or questions about this you should remember that some regions/countries may use different rules for which this answer may not apply.
References and further reading:
http://es.wikipedia.org/wiki/Wikipedia:Consultas/Consultas_ling%C3%BC%C3%ADsticas
http://www.fundeu.es/consultas-B-buenos-dias-buenas-tardes-2146.html
http://es.answers.yahoo.com/question/index?qid=20070622014500AAaMy6Y
As a side note, also something that should be taken into account when answering questions is that just because something is applicable in one country/region doesn't mean that is the rule and vise versa, the rule not always applies for all countries.
