GIVE ANSWERS - Español

It is because the gender of the name.

- Buenas tardes/noches. Both are used in fem plural so they add to the adjetive -as (a for femenim and s for plural)

- Buenos días. is masc plural so add -os (o for masc and s for plural).

- You also can find expresions where the adjetive has no gender added (mostly when no name is used or some cases instead of using masculine).

El buen comer (no name).

Hace un buen día (but Hace un día bueno para ir a caminar). Also "día" is an exception it ends in A but is masculine.

Es un buen hombre (but Es una buena mujer o buena persona).

bueno-good, good night, buenas noches plural and femenim, que tengas un buen día- have a good day (masculine) because the word día have a little accent on the top of the i, i in spanish is a masculine, buenos dias good morning greeting somebody in the morning,