From Robert Ford, Song Histories (William Hodge, 1900, p. 10):

It is a curious fact that in every mixed gathering--yea, on all occasions where "Auld Lang Syne" is sung in concert--the last line of the chorus is invariably rendered--

             For the days of auld lang syne.

although it is never printed that way, and the extra words are not necessary for the sake of the tune.