Thanksgiving: should be offered: Upon the Completion of Great Undertakings
Source: TTT
Nehemiah 12:31, 40
Then I brought the leaders of Judah up on the wall, and I appointed two great thanksgiving choirs. One was to proceed along the top of the wall to the right, toward the Dung Gate. / The two thanksgiving choirs then stood in the house of God, as did I, along with the half of the officials accompanying me,