unless you have notice them an earlier time. Anyway, you can keep you key until the last momnet. Last time when I moved out, the super didn't allow me to move in the moring of the first day, he said the next tenant want to move in at the end of the month, say August 31. I handed in my key until 23:50 of August 31. Nobody can force you out, since you already pay your rent for that day.