8th day of the month
Refers to the eighth day of a month. Often used with month names or in date expressions.
会議は3月8日です。
The meeting is on March 8th.
8日までに提出してください。
Please submit it by the 8th.
eight days
Indicates a duration of eight days. Often used with counters or in expressions of time span.
夏休みは八日間あります。
Summer vacation is eight days long.
あと八日で旅行に出発します。
We leave for the trip in eight days.
There is a meeting on the 8th of next month.
It took eight days to finish this task.