good ˈday especially British English old-fashioned used to say hello or goodbye, especially in the morning or afternoon:
I must get back. Good day to you. [TahlilGaran] Dictionary of Contemporary English ▲
good day interj. Hello or goodbye.
■ Used as a formal greeting or salute when you meet or leave someone during the day.
Miss Rogers said, "Good day!" when she met her friend on the street. Mr. Lee said "Good day!" and left the office. [TahlilGaran] English Idioms Dictionary ▲