Buenos vs Buenas
Why to you say BuenOS días and BuenAS noches
3 Answers
Días is an exception and is masculine, even though its singular día ends in a. So it does take OS. Noches is feminine so it does take the AS ending.
buenons días because "día" is a masculine noun, so it gets a masculine adjective
buenas noches because "noche" is a feminine noun, so it gets a feminine adjective
Have you tried the dictionary?