why is it when saying "Good morning" and "good night" you spell bueno differently?

Question

why is it when saying "Good morning" and "good night" you spell bueno differently?

Buenos dias
Buenas noches

p.s. I do know about the feminine - masculine articles, I'm just not sure how it applies in these two phrases.

Answer 1

Thank you for asking. I have corrected that error more times than any other error except using a comma before the in commenting on the lessons.

Good in this context is an adjective. It modifies either the noun days or nights.

Answer 2

The adjectives and the nouns must agree in quantity and gender.

dias is masculine plural, so it needs buenos, which is also masculine plural.
noches is feminine plural, so it needs buenas, which is also feminine plural.