The 20th day of a month (date).
今月の２０日は友達の結婚式です。
The 20th of this month is my friend's wedding.
二十日は給料日だから銀行が混む。
The 20th is payday, so the bank gets crowded.
８月２０日は僕の誕生日だ。
August 20th is my birthday.
A period of twenty days (duration).
夏休みは２０日間の予定です。
The summer vacation is planned to be twenty days.
その薬は服用してから約２０日で効果が出ると言われた。
They said the medicine takes about twenty days to take effect after starting it.
工事は約２０日で終わる見込みだ。
The construction is expected to finish in about twenty days.