buenos, buenas

Question

In Learn Spanish 1 Greetings and Introductions the flashcards show buenos días (good morning)
and buenas noches (good evening), Why is there an "o" in buenos and an "a" in buenas? How important is it to pronounce the "o" and the "a" when saying good morning and good evening'

Answer 1

sometimes Mexican people simply say "buenas" as a form of an abbreviated greeting
when what they are understood to be saying is - "buenas tardes" or "buenas noches"

Answer 2

agreement

Above is a link discussing Spanish concordancy or matching gender and number between parts of speech.

Answer 3

Buenos días = Masculine plural (Los días)
Buenas noches = Feminine plural (Las noches)