Why is this called "Mizmor l'Sodah"?
Rashi: It is proper to say it with a Korban Todah 1 .
Radak: Thank Him for the Chesed that He did with you.
Malbim: It is thanks for a miracle.
Rema (OC 51:10): We omit Mizmor l'Sodah on days when one may not bring a Todah - Shabbos, Yom Tov, Pesach, Erev Pesach and Erev Yom Kipur.
What is the meaning of "Hari'u Lo Kol ha'Aretz "?
Radak: The entire land should rejoice over your Simchah.
Malbim: When He conducts via nature, He is like a king who is closed in his palace and conducts [the country] via his officers and advisors. At the time of a miracle, He is like a king who leaves his palace and goes among the nation to personally supervise their affairs. Then, they all blow a great Teru'ah to declare him king.