Refers to the third day of a month or a period of three days. For instance, you might say 'stay for three days' or 'the third day of July'. It's a common way to express dates and durations.

月(つき)の三(みっ)日(か)目(め)、または「三(みっ)日(か)間(かん)」のように三(みっ)日(か)間(かん)を数(かぞ)えるときに使(つか)われます。