8th day of the month
Refers to the eighth day of a month. Often used with month names or in date expressions.
The meeting is on March 8th.
Please submit it by the 8th.
eight days
Indicates a duration of eight days. Often used with counters or in expressions of time span.
Summer vacation is eight days long.
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.