What day is discussed?
Radak: The day that the land will be destroyed.
What is the significance of the sun in the afternoon?
Rashi: When they will have great Shalom, their demise will come suddenly. Chazal said (Mo'ed Katan 25a) this is the day of Yoshiyah. He died via a sword of Shalom. There is no greater sword of Shalom than Pharaoh Necho. He told Yoshiyah "Mah Li va'Lach Melech Yehudah Lo Alecha Atah ha'Yom (I merely want to pass through your land) Ki Im El Beis Milchamti" (Divrei ha'Yamim II, 35:21) - (Ta'anis 22b). Malchus Beis David is compared to the sun - "v'Chis'o cha'Shemesh Negdi" (Tehilim 89:37).
Radak: This is a Mashal for the great affliction that they will have - "v'Kadar Aleihem ha'Yom" (Michah 3:6).
Malbim: This is the third illustration of the punishment. There will be darkness in their land, both from above (the sun will set - this is a Mashal for Hashem departing from them, and their Mazal and success will darken) and below - "va'Hachashachti la'Aretz." Below there will be cloud and darkness. This is a metaphor for making their counsel foolish and making their Gevurah stumble. Both will be supernatural.
