noun
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.
noun
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.
Explicitly marks a duration of eight days, while 八日 can mean either the date or the duration depending on context.
Compound of 八 (ya, 'eight') and 日 (ka, 'day'), with a regular sound shift from yaka to yōka.