John Gill Commentary Nehemiah 8:14

"And they found written in the law, how that Jehovah had commanded by Moses, that the children of Israel should dwell in booths in the feast of the seventh month;" — Nehemiah 8:14 (ASV)

And they found written in the law which the Lord had commanded
by Moses
The children of Israel, to be observed by them; either by hearing it read the day before, or by conversation with Ezra, they perceived it was enjoined in the law, particularly in (Leviticus 23:39–43) (Deuteronomy 16:13)

that the children of Israel should dwell in booths, in the feast of the
seventh month :
which was the same month, and this the second day of it, and therefore the time drew near for keeping it; for it was to begin the fifteenth.