From (the Book of) Nehemiah 8:2
So on the first day of the seventh month Ezra the priest brought the Law before the assembly, which was made up of men and women and all who were able to understand.
Which Nehemiah 8:10-12
Nehemiah said, "Go and enjoy choice food and sweet drinks, and send some to those who have nothing prepared. This day is holy to our Lord. Do not grieve, for the joy of the Lord is your strength."
The Levites calmed all the people, saying, "Be still, for this is a holy day. Do not grieve."
Then all the people went away to eat and drink, to send portions of food and to celebrate with great joy, because they now understood the words that had been made known to them.
I'm curious if this is the telling of a particular feast day important in Judaism or if it was something more general.