1.
nine days
A period of nine days.
九日 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:
- 九日かかった: it took nine days (duration)
- 九日に届く: arrives on the 9th (date)
- Add 間 to emphasize duration: 九日間
DAYS OF THE MONTH (native readings, 1st-10th):
From the 11th onward, regular Sino-Japanese readings are used (e.g. 十一日), except 二十日 (20th).