Buon and Buona for buon giorno and buona giornata
Why does 'good' change from masculine to feminine when saying buona giornata? Day (giorno) appears to be a masculine noun normally.

Question

Answer

"Giorno" and "giornata" are different nouns with different gender – "giorno" is indeed masculine, but "giornata" is feminine. For the semantics see https://blogs.transparent.com/italian/giorno-or-giornata/

Answer

Hi Tierney,
"Buon giorno" means "good morning" whereas "Buona giornata" means "have a good day" .
Buona giornata is used to wish a good day as a time when people are busy doing things.
Buon giorno is used in the morning. Then we have "buon pomeriggio, buona sera, buonanotte".
As well as "buona giornata" we use "Buona serata" to wish a nice evening.