This feast, which is not specifically mentioned as such in the OT, took place at the observance of the new moon in the month of Tishri (September/October). It signalled the end of the agricultural year.

Leviticus 23:24-25

It was to be a day of rest, commemorated with the sounding of trumpets

Numbers 29:1
Leviticus 23:25

Offerings were made to God

Numbers 29:2-5
Nehemiah 8:2-6

Celebrated after the exile

See also

4408 balm
5056 rest