Refers to the second day of a month or a period of two days. For example, '2nd of April' would use this. It's part of the traditional Japanese counter for days.

月(つき)の二(ふつ)日(か)目(め)、または「二(ふつ)日(か)間(かん)」のように二(ふつ)日(か)間(かん)を数(かぞ)えるときに使(つか)われます。