Masechet Soferim

Question: Why do we read Shir HaShirim on Pesach? Also, why do we generally read it on the Shabbat of Chol HaMoed as opposed to the first days of Pesach? Finally, why don’t we recite a blessing over the reading of Shir HaShirim as we do for Megillat Esther? Menachem (Via E-Mail)