Bonsoir vs. bonne soirée
The French expressions 'bonsoir' and 'bonne soirée' both relate to wishing someone a good evening, but they are used in different contexts and at different times.
Bonsoir
'Bonsoir' is used as a greeting when meeting someone in the evening.
Bonsoir, comment allez-vous?
(Good evening, how are you?)
Bonsoir Madame, bienvenue au restaurant.
(Good evening Madam, welcome to the restaurant.)
Bonne soirée
'Bonne soirée' is used when parting ways in the evening, wishing someone to have a good rest of their evening.
Au revoir et bonne soirée!
(Goodbye and have a good evening!)
Merci pour le dîner, bonne soirée à vous.
(Thank you for dinner, have a good evening.)
Summary
While 'bonsoir' is used as an evening greeting when meeting someone, 'bonne soirée' is used when saying goodbye, wishing the person a pleasant remainder of their evening. 'Bonsoir' starts an interaction, while 'bonne soirée' concludes it.