When is "that day"?
Rashi: When sinners will cease.
Radak: When the 10 tribes will be exiled.
Radak (6, from Sanhedrin 111b and Targum Yonasan): In the days of Mashi'ach.
Malbim: It is when Efrayim will fall.
Why does it say "la'Ateres Tzvi veli'Tzfiras Tif'eres"?
Radak: This is the opposite of "Ateres Ge'us" (3). Tzefiras Tif'eres is the same as Ateres Tzvi; the matter is repeated in different words to strengthen it. Tzefirah is going around. Therefore, an Atarah (crown) or Mitznefes ha'Rosh is called Tzefirah, because it goes around the head. A basket receives Tum'ah from when one makes two Tzefiros (Kelim 16:3), i.e. reeds wrapped around it. Kel will be honor and glory for them in the days of Chizkiyah.
Radak (6, from Sanhedrin 111b): Hashem will be a crown on the head of every Tzadik, for those who do Tzivyono (His will) ul'Metzapin Tif'arto (and look forward to His glory).
Malbim: La'Ateres Tzvi is in contrast to "Ateres Ge'us" (3). Veli'Tzfiras Tif'eres, dawn light that increases until it is proper daylight, is in contrast to "Tzitzas Novel" (4).
Who are "She'ar Amo", for whom Hashem will be Ateres Tzvi and Tzefiras Tif'eres?
Sanhedrin 111b: It is one who makes himself like Shirayim (leftovers), and "l'Ru'ach Mishpat" (6, one who judges his Yetzer, i.e. desires) "la'Yoshev Al ha'Mishpat" (one who judges a case to its ultimate truth) "veli'Gvurah" (one who overcomes his Yetzer) "Meshivei Milchamah" - (they answer and debate in the war of Torah) "Sha'arah" (Chachamim who come early, when they open the gates of Batei Kenesiyos and Batei Medrash and leave late, when they lock them).
Rashi: The Tzadikim who will remain.
Radak, Malbim: They are Yehudah and Binyamin, who remained when the 10 tribes were exiled in Chizkiyah's sixth year.
