Why does it say that he was not like Malchei Yisrael before him?
Rashi (from Seder Olam 22): From the days of Yaravam ben Nevat there were guards on the roads (Radak - on the borders 1 to stop people from ascending to Yerushalayim for the Regalim. Hoshe'a abolished the guards 2 , for Yaravam's golden calves were removed during the first two exiles. In the first exile, 'va'Yikach Es Iyon v'Es Dan 3 ' - the calf in Dan was exiled. In the second exile, of Gad and Reuven, the calf in Beis Kel was taken, to fulfill "Gam Oso l'Ashur Yuval" (Hoshe'a 10:6). Because the guards were abolished, and even so people did not ascend for the Regel, the decree for exile was sealed in Hoshe'a's days. Until now, the kings were to blame; now, the people were liable - "Ki Atah Hiznik Efrayim Nitma Yisrael" (Hoshe'a 5:3) - now your evil is revealed.
I.e. between Malchus Yisrael and Malchus Yehudah. (PF)
Radak citing Ta'anis 30b. This was on Av 15. Therefore, Yisrael make a Yom Tov on this day. (The Gemara gives also other reasons why it is a Yom Tov.)
Also Midrash ha'Gadol (Devarim 29:27) cites the verse above (15:29) to say so. That verse says that Tiglas Pil'eser took Iyon, Avel, Beis Ma'achah, ha'Galilah Kol Eretz ha'Naftoli", but does not mention Dan! Another verse (15:20) says that Ben Hadad struck Iyon, Dan, Avel, Beis Ma'achah
