Which day is discussed?
Radak: The day when the above wonder will be.
What is "Ohr Yekaros v'Kipa'on"?
Pesachim 50a #1: This is light (Aruch - of Torah), which is Yakar (dear, or scarce) in this world, and Kafuy (prevalent) in the world to come.
Pesachim 50a #2: These are [laws of] Tzara'as and Tum'as Ohel, which are Yakar in this world, and Kafuy in the world to come.
Pesachim 50a #3: These are [rich] people who are Yekarim (important) in this world, but they are [not righteous, hence they will be] Kefuyim (light, unimportant) in the world to come.
Rashi citing Targum Yonasan: There will not be light, rather, ice. The light will not radiate; it will be Yekaros and Kipa'on. Yekaros is like "ki'Ykar Karim" (Tehilim 37:20), like light seen in the morning on the mountains; it fades away. Kipa'on is frozen, dark and thick like ice. This is like "vecha'Gevinah Takpi'eni" (Iyov 10:10).
Radak: This is a Mashal. The light will not be clear that day. i.e. Ohr Yekaros, like "v'Yare'ach Yakar Holech" (Iyov 31:26), and not Ohr Kipa'on - frozen and thick, like darkness. I.e. the light of that day will not be like light, and not like darkness. I.e. there will not be respite, and not affliction. Both of them will be, like it says "Lo Yom v'Lo Laylah" (7).
Malbim: It is like of the sun and moon. They will cease to illuminate - "Lo Yihyeh Lecha Od ha'Shemesh l'Or Yom ul"Nogah ha'Yare'ach Lo Ya'ir Lach v'Hayah Lach Hashem l'Or Olam" (Yeshayah 60:19). There will not be physical light - not light of the sun, which is its own (Ohr Yekar), and not light of the moon and stars 1 , which merely reflect from the light that they receive from the sun.
Chazal refer to the other planets as stars. Real stars emit their own light, like the sun. (PF)
