Used as a suffix to indicate the day of the month (one reading of 日 pronounced 'ka' in words like 三日, 十日).
今日は三日（みっか）だから月始めの手続きが多い。
Today is the 3rd, so there are many month-beginning procedures.
彼の誕生日は十日（とおか）だ。
His birthday is on the 10th.
二十四日は二十四日（にじゅうよっか）と書く。
The 24th is written as 'nijūyokka' (24th).
A counter/readings used to count days (forms like 五日, 二日, where 日 is read as 'ka' in certain day-count words).
旅行は五日（いつか）間の予定です。
The trip is scheduled for five days.
彼女は二日（ふつか）で戻ってくると言った。
She said she'll be back in two days.
合宿は三日（みっか）間行われる。
The training camp will be held for three days.