What is the significance of the sun darkening?
Rashi: This is to shame those who bowed to it.
Radak (3): The unnatural lightning, darkening of the sun and the moon turning to blood, the Meforshim explained like the simple meaning. These wonders will occur then. Ibn Ezra said that darkening of the luminaries is a sign of great wars; it will be before the day when Gog and his allies will be stricken.
Radak (3, citing the Rambam): These wonders are a Mashal for the afflictions that will come against the nations and the wars. So it said about the locusts "Lefanav Ragzah ha'Aretz Ra'ashu Shamayim Shemesh v'Yare'ach Kadaru v'Chochavim Asefu Nagham" (2:10), and it says similarly about the fall of Gog and Magog 1 (4:14).
Malbim: The sun influences light. It will transform to darkness due to the burning and smoke - "Hinei ha'Yom Ba Bo'er ka'Tanur" (Mal'achi 3:19).
Radak: If so, it should say b'Vo Yom Hashem, and not "Lifnei Vo Yom Hashem"! Perhaps he means that the affliction will be for Yisrael. Light will be for darkness before the day that Gog and Magog fall. He will come with many nations and fight against Yerushalayim; "half the city will be exiled"; "Lo Yihyeh Ohr Yekaros v'Kipa'on" (Zecharyah 14:2, 6). The day that Gog and Magog will fall is "Yom Hashem ha'Gadol veha'Nora" (Mal'achi 3:23). This is like it says about the locusts "Ki Gadol Yom Hashem v'Nora Me'od" (2:11).
What is the significance of the moon turning to blood?
Radak: This is darkening of the moon. When it is totally dark, it will be black. When it is partially black it will be red like blood.
Malbim: The moon is a Mashal for moisture. It will turn to blood, in place of water. I.e. the orders of the Ma'arachah will change, to show the killing, burning and destruction.
What is "Yom Hashem ha'Gadol"?
Radak: It is the day when Gog, Magog and the nations with them will be stricken.
Malbim: It is the great day of judgment.
