One day, One afternoon
Incorrect: I met her a day in June.
Correct : I met her one day in June.
Incorrect : He saw me an afternoon in July.
Correct : He saw me one afternoon in July.
Note: “One' cannot be replaced by “a” or "an" when it means 'a certain'. In this sense "a/an' cannot be used with day, night, morning, afternoon, and evening.
