Why did he make the Chag in the eighth month?
Radak: (a) This was to distance Yisrael from the Avodah of Yerushalayim. (b) If some Yisraelim will want to go to Yerushalayim for Sukos 1 , they will have returned to their homes, and they can come to Beis Kel for the Chag amidst fear of the king.
Malbim: It was on the 15th of the month, like the Torah's Chag, but in a different month, to make a great distinction.
Or, if they ascended before hearing Yaravam's command not to ascended. Refer to 12:27:2:1*. (PF)
Why does it say "k'Chag Asher bi'Yhudah"?
Radak: Like the Chag in Yehudah in the seventh month, so was the Chag in Beis Kel in the eighth.
Malbim: He called it Sukos, to revile and anger [Hashem].
Why does it say "la'Agalim" (plural)? There was only one Egel in Beis Kel!
Radak: There was also one in Dan. It mentions Beis Kel, for Yaravam went there to offer and the Navi came to him there.
Malbim: In Beis Kel they served also the Egel in Dan. Also one who vowed to bring to the Egel in Dan would bring in Beis Kel, so the Avodah and Korbanos would be in one place, similar to Yerushalayim.
