Why is it that we read on Shabbos Chol Hamoed Pesach the Torah portion that is read on a fast-day?
Almost the same less elaborate different source: http://www.angelfire.com/pa2/passover/faq/chol-hamoed-pesach.html
The recital of the Thirteen Attributes of Mercy serve as the key to gaining the compassion of G-d toward the Hebrews/Jewish people. Although the time of Chol HaMoed is a partial or semi holiday time period, it is still a time of holiness due to its partial or semi holiday status. Therefore, the purpose of mentioning the Thirteen Attributes of Mercy during the Chol HaMoed period is to remind us that even though we are able to perform certain forms of work in this time period, we are always dependent on the mercy of G-d and on G-ds' interaction with all things in this world. Without the compassion of G-d, we would not even have the opportunity to be able to perform any of our daily activities. Therefore, the lesson we learn from this Chol HaMoed Torah reading is that we are always available for use by G-d and that we must always aim to better ourselves in all phases of life so that we continuously merit the mercy and compassion of G-d.