What is the "Bamah Gedolah"?
Rashi: It is the copper Mizbe'ach that Moshe made 1 in the Midbar. It was fixed in Shilo; when Shilo was destroyed, it was moved to Nov. Nov was destroyed in the days of Sha'ul, and it was moved to Giv'on. Radak - it is not called Gedolah due to its size, rather, due to its higher level than the others. Fire descended to it from Shamayim on the eighth day of Milu'im; the fire was constantly on it. Other Bamos, every commoner made as he saw proper. They are called Bamos Ketanos, for they are less esteemed.
Radak: I.e. Betzalel made it according to Moshe's command. Divrei ha'Yamim II, 1:6 explicitly says that Shlomo offered on the copper Mizbe'ach in the Ohel Mo'ed.
When did Shlomo offer 1000 Olos?
Rashi: He did so every day. It is as if it says "Ma'aleh".
Radak: This is past tense, but it is written in the future conjugation. Many verses are like this. He did not offer them in one day, rather, before he returned from Giv'on to Yerushalayim. However, Zevachim 59b implies that he offered them at once (in one day).
Malbim: It was the day that he gathered all of Yisrael to Giv'on (refer to 3:4:3:1).
In Divrei ha'Yamim II, 1:3 it says that Shlomo gathered all of Yisrael to Giv'on at the time. Why did he do so?
Malbim: He told everyone that it is improper to offer on Bamos, rather, on the copper Mizbe'ach in the Ohel Mo'ed in Giv'on, on which was fire from Shamayim.
