good days - or is it goods days?

Adjectives don't just agree with masculine or feminine. They also agree for singular or plural. That means in French and Spanish you add an "s":

In Spanish, you start with "bueno" and "día". Add an "s" to each and you get:

Spanish buen día becomes buenos días (bweh-nohss dee-ahss).
Where'd the "o" in "buenos" come from? Remember, if an adjective ending in "o" comes before a noun, you drop the "o". But, when you make it plural, i.e. "buenos," it doesn't end in "o". It ends in "s". Tricky. I know. Just say "buen día" and "buenos días" a few times and you'll remember the rule, even if it doesn't make any sense.

French bon jour becomes bons jours.
Don't pronounce either "s". Just say "bohn zhoor". Easy, huh?

In Italian, you start with "buono" and "giorno". Change the "o"s to "i"s and:
Italian buon giorno becomes buoni giorni (bwoh-nee jor-nee).
Like Spanish, you start with "buono" because it's not in front of a noun yet and you only drop the "o" if it is definitively what's going to come before the noun.