What is "that day"?
Malbim: When Melech Ashur will return from Egypt and set his face to conquer Eretz Yehudah.
What is the meaning of "Yishrok Hashem"?
Rashi: He will hint to them to gather.
Radak: It is as if He will whistle for them to come. I.e. He will arouse their spirits. So it says above "v'Sharak Lo mi'Ketzei ha'Aretz" (5:26).
Radak citing Targum Yonasan: Hashem will eradicate a nation like bands of camps of Giborim.
What are the fly and the bee?
Rashi: The fly hints to troops numerous like flies from Egypt who will come with Sancheriv. The bee hints to strong troops that sting like bees.
Radak: Flies and bees gather in great numbers in a moment. So the troops of Egypt and Ashur will gather and come quickly - "Meherah Kal Yavo" (5:26). Similarly, it says "Sabuni chi'Devorim" (Tehilim 118:12), "Ka'asher Ta'asenah ha'Devorim" (Devarim 1:44).
Malbim: The fly is Egypt, and the bee is Ashur - the latter is stronger.
Why does it say "bi'Ktzei Ye'orei Mitzrayim"?
Radak: The Katzinim (officers) of Egypt will come with Melech Ashur when he comes against Eretz Yisrael. It mentions the rivers, because everywhere farmers fix the rivers to irrigate their land. Even farmers will abandon their work and come, amidst their great desire to destroy Eretz Yisrael.
