day: the stretch of daylight, a single dated day, or a span of time, including the appointed day when God acts in judgment and salvation.
This is the ordinary word for a day, and most of the time it means exactly what you expect: the daylight hours when work can be done, or one dated day on the calendar. From there it stretches naturally to a length of time measured in days, and then, in the plural, to a whole season or era ("in those days," "the last days").
What gives the word its weight in the New Testament is a use carried straight over from the Hebrew prophets: the Day, the single appointed day when God steps into history to judge and to save. "The day of the Lord," "the day of judgment," "that day," and "the last day" all use this one word, and the reader is meant to hear the calendar word turned into the name of a fixed, coming moment.
The senses are not separate words; they shade into one another. The same word that counts off the days of a journey names the day a person stands before God. Context, not the word itself, tells you whether a verse means the daylight, a date, an era, or the Day.