• 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.albaiges.com/cronologia/cuandoempiezatarde.htm
• http://forum.wordreference.com/showthread.php?t=975209
• 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.

• 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.albaiges.com/cronologia/cuandoempiezatarde.htm

• http://forum.wordreference.com/showthread.php?t=975209

• 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.

• 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)

• 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.albaiges.com/cronologia/cuandoempiezatarde.htm
• http://forum.wordreference.com/showthread.php?t=975209
• 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.