Neh 8:14

“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 Lev 23:39 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.”

“In the feast of the seventh month - That is, the feast of tabernacles, which was held in commemoration of the sojourning of their fathers in the wilderness after they had been delivered from the Egyptian bondage. Now, having been delivered from the Babylonish captivity, and the proper time of the year occurring, it was their especial duty to keep the same feast.”