Fetching details from the dictionary...

one day
all day (long); the whole day; from morning till night
first day of the month
「1日」 has another reading with different meanings.
1日に三回ご飯を食べる。
I eat three meals a day.
1日じゅう家にいたかった。
I wanted to stay home all day.
1日だけの休みでも嬉しいです。
Even just one day off makes me happy.