The 5th day of the month (a calendar date).
今日は5日だから、家賃を払わないといけない。
Today is the 5th, so I have to pay the rent.
私の誕生日は五日です。
My birthday is on the 5th.
来月の五日は友達の結婚式があります。
On the 5th of next month there's a friend's wedding.
A period of five days (a duration of five days).
出張で五日間東京に行ってきます。
I'm going to Tokyo on a business trip for five days.
その休暇は五日間十分リフレッシュできた。
That vacation for five days was enough to refresh me.
図書館は祝日のため五日間休館します。
The library will be closed for five days because of the holidays.