Learn French: Bon week-end, Monsieur ! - Have a good weekend, sir!

In French, you do not need a verb to wish someone well in this context. You literally just say Bon week-end, meaning "Good weekend." You will see this same pattern in other everyday wishes like Bonne journée (Have a good day) or Bon voyage (Have a good trip).

Is week-end a masculine or feminine word?

It is masculine. That is why the sentence uses the masculine adjective bon instead of the feminine bonne. As a helpful tip, French almost always treats words borrowed from English as masculine nouns!

Why is there an extra space before the exclamation mark?

In French writing and typing rules, you must put a space before "two-part" punctuation marks. This includes exclamation marks (!), question marks (?), colons (:), and semicolons (;). English never adds this space!

How do you actually pronounce Monsieur?

The spelling of Monsieur is a very old exception to normal French reading rules. It is pronounced roughly like "muh-syuh." You completely ignore the n in the middle and the final r when speaking.