I Kings 8:2

וַיִּקָּהֲלוּ אֶל הַמֶּלֶךְ שְׁלֹמֹה כָּל אִישׁ יִשְׂרָאֵל בְּיֶרַח הָאֵתָנִים בֶּחָג הוּא הַחֹדֶשׁ הַשְּׁבִיעִי

All the men of Israel assembled themselves before King Solomon at the holiday in the month Ethanim, which is the seventh month.

The Radak explains that this was before the holiday of Succos. They actually gathered seven days before Succos and were there for a total of two weeks as we see later, in verse 65 – one week to set up the Temple for the holiday and a second week to celebrate it. (Don’t be confused by the name “Ethanim.” It’s Tishrei. The names for the months were different before the first exile. Other early month names in Tanach include Ziv and Bul in I Kings 6, better known to us as Iyar and Cheshvan.)