On the first day of the month, they celebrated The Feast of Trumpets.
On the tenth day of the seventh month, they observed The Day of Atonement.
Then the Bible adds this for the fifteenth day of the seventh month,
“Speak to the children of Israel, saying: ‘The fifteenth day of this seventh month shall be the Feast of Tabernacles for seven days to the LORD” (Leviticus 23.34).
Therefore, once a year, every Israelite was to dwell in a tabernacle, a tent, for seven days during the seventh month to remember how they had spent their time in the wilderness after the Lord delivered them from Egypt.