(ここのか)

ここのか
noun
nine days, ninth day
1. nine days
A period of nine days.
The trip took nine days.
九日(ここのか)(かん)(やす)んだ
I took nine days off.
It will be completed in nine days.
九日(ここのか)ぶり()った
We met for the first time in nine days.
九日(ここのか)()った
I waited for nine days.
2. ninth day of the month
The ninth day of any month.
九月(くがつ)九日(ここのか)何曜日(なんようび)ですか
What day of the week is September 9th?
There is a meeting on the 9th.
It will arrive on May 9th.
九日(ここのか)予定(よてい)ある
I have plans on the 9th.
The 9th of next month is free.

九日(ここのか) has two uses: it counts a duration of nine days, and it indicates the 9th day of a month. The reading ここのか comes from the native Japanese number system, not the Sino-Japanese system.

DURATION VS DATE:

DAYS OF THE MONTH (native readings, 1st-10th):

From the 11th onward, regular Sino-Japanese readings are used (e.g. 十一日(じゅういちにち)), except 二十日(はつか) (20th).