1. A polite greeting used in the late evening or night, typically after sunset or from evening onwards
As the sun set, Maria said 'buenas noches' to her colleagues before leaving work.
Quando o sol se pôs, Maria disse 'boa noite' aos seus colegas antes de sair do trabalho.
2. A farewell expression used when departing in the evening or night
Before going to bed, he wished everyone 'buenas noches'.
Antes de dormir, ele desejou a todos 'boa noite'.