Learn French: Au revoir, Monsieur, et bonne journée ! - Goodbye, sir, and have a good day!

Because the word for day here, journée, is a feminine noun. In French, describing words (adjectives) have to match the gender of the noun they describe. The masculine form is bon and the feminine form is bonne.

I learned that "jour" means day. Why do we use "journée" here?

Jour is mostly used to talk about a day as a simple unit of time, like when you are counting how many days have passed. Journée emphasizes the whole duration and experience of the day from morning to evening. Because you are wishing someone a good continuous experience throughout their day, you use bonne journée.

How do you actually pronounce "Monsieur"?

The spelling is an old exception and doesn't match standard pronunciation rules! It sounds roughly like "muh-syuh". You do not pronounce the "n" in the middle, and the "r" at the very end is completely silent.

What does "au revoir" literally translate to?

It literally translates to "to the seeing again". The word au means "to the", and revoir is the action word (verb) meaning "to see again". It is the traditional French way of saying "until we meet again".