The 24th day of the month (date). Usually read 'nijuu-yokka', though 'nijuu-yon-nichi' may be used in some contexts.
支払いの締め切りは今月の２４日です。
The payment deadline is the 24th of this month.
二十四日のイベントにはたくさんの人が来るだろう。
Many people will likely come to the event on the 24th.
A period of 24 days (twenty-four days).
実験は合計で２４日続いた。
The experiment lasted a total of 24 days.
商品の返品は受け取りから２４日以内にしてください。
Please return the product within 24 days of receipt.