by the day
Per day; at a daily rate. You can't afford to miss work when you get paid by the day. I'm renting this hotel room by the day because I could get my next travel assignment at any time.
by the day
one day at a time. I don't know when I'll have to leave town, so I rent this room by the day.
by the day
Also, by the hour or week or month or year . According to a specific time period, as in I'm renting this car by the day, or He's being paid by the hour. This usage generally describes some kind of rate. [1400s]