If días ending is a which is feminine then why is it buenos días, instead of buenas días?
I am having a misunderstanding with this greeting. Maybe if I understand this theory then I can begin to understand others like it.
6 Answers
very true...you have to be careful with the "a" rule...not all words ending in -a are feminine...just as not all words ending in -o are masculine. Día is a prime example of one of these "rebels" as I like to call them. El día becomes los días...and would then make buenos días...an opposite examle would be la mano...the hand. Allthough it ends in an -o it is feminine. This is why you should always learn the gender with the noun...just to be on the safe side... 
My teacher used to say "All the 'dad's are feminine and all the 'ma's are masculine." It's a kind of cute way to remember the ironic gender swap. Because most words that end in "dad" are feminine, and everyone has already mentioned the "ma" ones.
Just be aware of these exception, after a while you get used to them and they will seem normal.
Here is a list of common nouns which end in -a and are masculine:.
el problema el telegrama el programa el mapa el sistema el poema el día el tema el clima el idioma el sofá el planeta
Many nouns that end in -ma are masculine. Notice that eight of the twelve nouns listed above end in -ma.
el telegrama el programa el problema el sistema el poema el idioma el clima el tema
A few nouns that end in -ma are feminine, such as la cama and la pluma.
the words which ends with "a" aren't always femenino... it is el día , not la día ... there are some exceptions..
There are no absolute rules. There are general rules with exceptions. That's life.
I don't understand that one either.
