Hail and Farewell (a translation of ave atque vale, last words of the poem Catullus 101) is a traditional military event whereby those coming to and departing from an organization are celebrated.
At the same time it is a welcome to those who are joining and introduces them to the special history and traditions of their new organization.
For the United States Army, a Hail and Farewell is most often celebrated at a formal dining in when there is a change in command.
There are no official requirements outlined by the United States Army to have a Hail and Farewell celebration.
[1] The United States Navy, on the other hand, has specified that by custom the ship's officers must give a formal dinner when their new captain arrives.