Buenos /Buenas

[Varian]

Why is it Buenos días but buenas tardes and buenas noches?

[Julvenzor]

Simply because "día" is a masculine noun due to an etymological reason. I read a pair of days ago una explicación muy precisa tocante a esto:

The real question here is why día ‘day’ is masculine even though it ends with -a, the Spanish feminine ending par excellence. I have been looking into words like this lately; día ‘day’ is the most frequent of them. Día is masculine because it comes from the Proto-Indo-European root *diéus, meaning ‘Sky-god’ (a masculine deity) or ‘daytime sky’. It ended up with a final -a mostly because its immediate Latin progenitor, diēs, was the only masculine word in Latin’s ‘fifth declension’ noun class. (Doesn’t “The Fifth Declension” sound like a good name for an amateur rock band composed of linguists, like Stanford’s “Dead Tongues”?) Other nouns in this category also ended in -ēs, or even -iēs. They included effigiēs ‘effigy’, rēs ‘thing’, and speciēs ‘sight, view; shape, form’.
The ending of several fifth declension nouns changed from -iēs to -a as Latin evolved into Spanish. Speciēs, for example, became especia ‘spice’. Other examples include Latin materiēs, which evolved into Spanish materia ‘matter, substance’ and madera ‘wood’, and rabiēs, the source of Spanish rabia ‘rage’.
These changes were part of a broader tendency to extend the explicit ‑a marker to nouns that were already feminine, such as infanta ‘princess’ (from Latin infante) and señora ‘madame, lady’ (from seniōre). Likewise, many masculine nouns acquired a freshly-minted -o ending. Examples include Spanish pájaro ‘bird’, from Latin passare and corcho ‘cork’, from Latin cortice. Ralph Penny refers to these changes as ‘hypercharacterization’.
For diēs the change to ‑a created a conflict between the noun’s gender and its ending. It was probably abetted by the fact that diēs was sexually ambiguous. While normally masculine, diēs wastreated as feminine when used in the sense of ‘appointed day, deadline’. In other words, gender confusion has been built into the word from the get-go. Plus ça change…

Source: http://spanishlinguist.us/2014/12/why-is-dia-masculine/

PD: Me percaté a posterior de la mezcolanza lingüística y decidí dejarlo tal cual.

[Varian]

Thanks for the explanation