Refers to the fifth day of a month or a period of five days. This reading 'itsuka' is also part of the traditional Japanese way of counting days and is frequently used in daily conversation.

月(つき)の五(いつ)日(か)目(め)、または「五(いつ)日(か)間(かん)」のように五(いつ)日(か)間(かん)を数(かぞ)えるときに使(つか)われます。