I Kings 8:2

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

All the men of Israel gathered unto King Solomon on the holiday in the month Ethanim, which is the seventh month.

The Talmud (Rosh Hashana 11a) discusses why “Eisanim” – which means “mighty ones” – is the Biblical name for the month of Tishrei. Rabbi Eliezer says it is because the Avos – Avraham, Yitzchak and Yaakov – were born in that month. Rabbi Yehoshua says it’s because the month is made mighty through the many mitzvos of Rosh Hashana, Yom Kippur and Succos.