九日

1. Origins, structure, and basic meaning

• Structure: 九日 is the irregular date and duration expression read ここのか, rather than a direct on-reading of the number and day.
• Basic usage: 来月の九日 means the ninth of next month, while 九日かかった means that something took nine days.

2. Common pitfalls and similar expressions

• Date and duration
  • With a month it marks the ninth day as a date; with a duration expression such as かかる, it denotes nine days.
• Reading and 九日間
  • Both uses are read ここのか; 九日間 can make the duration meaning especially explicit.