Okay, so I'm no expert in spanish, but I do know this. You use buena with feminine words (la words) and bueno with masculine words (le words). Feminine words tend to end in -a but there are exceptions. Dia ends with -a but is a masculine word. So you say buenos dias not buenas dias.

Hi Harper, this is one of the most challenging things in Spanish, how to know when a word is feminine or masculine. Usually, in Spanish a feminine word ends in -a, and a masculine word in -o; however there are some exceptions of this rule and sadly "días" is one of them, so you said "Buenos días " instead of "Buenas días." There is no rule to remember what the exceptions are, so, you need to learn them all.

días is a masculine word, so you always must use buenos.