also: ごにち
5th day of the month
Used for dates, such as May 5th or the 5th of any month. The reading is always いつか in this sense.
来月の五日に会議があります。
There is a meeting on the 5th of next month.
五月五日はこどもの日です。
May 5th is Children's Day.
five days
Refers to a duration of five days. The reading is いつか; the irregular reading ごにち is rarely used.
この仕事は五日かかります。
This work will take five days.
五日間の旅行に行ってきます。
I'm going on a five-day trip.
It takes five days to finish this job.
I sent the letter five days ago.