What is the significance of "that day"?
Malbim: There were two groups of idolaters. (a) One totally denied the Creator of Shamayim and Aretz; they attributed everything to the idol. Another group admitted that there is a Creator of everything, but they denied His Hashgachah, and said that He abandoned the land to the rule of the stars. They thought that the idol will be an intermediary between them and Hashem; via it, they will receive influence from Hashem. On that day, both groups will depart.
What is the meaning of "Yish'eh"?
Rashi: He will turn to his Maker. This is like "va'Yisha Hashem El Hevel" (Bereishis 4:4).
Radak: It is to turn or to trust. If it is to turn, "Al" is like El. Chizkiyah returned Yisrael for the good, and eradicated idolatry from the Beis ha'Mikdash and the land. He taught to them that Hashem is Elokim; it is proper to serve Him and trust in Him.
Does this refer to all people?
Rashi: It refers to the Tzadikim that remain among them.
Why does it say "Osehu"?
Radak: They will understand that Hashem makes man and forms him.
Malbim: Everyone will recognize that there is a G-d who made him - the Creator of all, and not the idol.
Why does it say "Kedosh Yisrael"?
Radak: He sanctified Yisrael and separated them from the nations to be to Him, and to serve Him alone.
Malbim: This shows that all will believe in His Hashgachah, that He alone supervises, and not via an intermediary. "Kedosh Yisrael" hints to special Hashgachah Pratis via His clinging to Yisrael, via their Kedushah, to the point that His Kedushah is attributed to them.
