Meaning of the French Idiom "La Lune de Miel" - Honeymoon

The French expression "la lune de miel" is a sweet one (pun intended). "Le miel" means honey in French, so the literal translation is moon of honey. 

Originally, this expression refers to the lunar month which follows a wedding, a time when the newlyweds are crazy in love, and all is perfect and sweet, just like honey.

Just like in English, this expression is used to describe the honeymoon trip. In French, the typical expression for this trip is "le voyage de noces" = the trip of wedding, "les noces" being an older French term to say "le mariage" - the marriage (note, one R in French though), the wedding.

Ils ont passé leur lune de miel à Paris : comme c'est romantique !
They spent their honeymoon in Paris: how romantic!

We usually use partir (to leave) or passer (to spend) with this expression.

Tu es partie où en lune de miel?
Where did you spend your honeymoon?

Où avez-vous passé votre lune de miel?
Where did you spend your honeymoon?

As far as I know, we don't have a word for honeymooners. We'd say: "les gens qui vont en voyage de noces" - people that go on a honeymoon.

Figuratively, "la lune de miel" refers to the high point of a relationship and often implies that things went downhill after that. 

Moi qui pensais aimer mon nouveau boulot.... Je me suis disputée avec ma collègue hier. Finie la lune de miel ! Me who thought I liked my new job... I had an argument with my coworker yesterday. The honeymoon is over.