Thayer's Greek Lexicon

 1. the day, used of the natural day, or the interval between
sunrise and sunset, as distinguished from and contrasted with
the night
  1) in the daytime
  2) metaph., "the day" is regarded as the time for abstaining from
indulgence, vice, crime, because acts of the sort are
perpetrated at night and in darkness
2. of the civil day, or the space of twenty four hours (thus
including the night)
  1) Eastern usage of this term differs from our western usage. Any
part of a day is counted as a whole day, hence the expression
"three days and three nights" does not mean literally three
whole days, but at least one whole day plus part of two other
days.
3. of the last day of this present age, the day Christ will
return from heaven, raise the dead, hold the final judgment,
and perfect his kingdom
4. used of time in general, i.e. the days of his life.