1.
one day (duration)
A period of 24 hours or one full day.
2.
all day, the whole day
Throughout an entire day from morning to night.
一日 means 'one day' as a duration. Note the different reading: when meaning 'the first day of the month,' it's read as 一日. 一日中 means 'all day long.' Compare with other duration words: 二日間 (two days), 三日間 (three days).