also: ごにち
noun
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.
noun
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.
The reading いつか is a native Japanese word combining 五 (itsu, 'five') and 日 (ka, 'day'). The irregular reading ごにち is a Sino-Japanese reading rarely used in modern Japanese.