Original: In French "Belle de Jour" is an expression translating literally as "beauty of (the) day," as opposed to "femme de nuit" or "belle de nuit," women of the night (common references to prostitutes).
Simplified: In French "Belle de Jour" means "beauty of (the) day," as opposed to "femme de nuit" or "belle de nuit," women of the night (common references to prostitutes).