Wikipedia says; Davy sang 'Massa is a stingy man', from the repertoire of Dan Emmett, one of the stars of American minstrelsy, which contains the lines:

"Sing come day, go day
God send Sunday
We'll drink whiskey all de week
And buttermilk on Sunday"


This has been turned around to make the new chorus:

"Come day, go day
I wish in my heart it was Sunday
Drinkin' buttermilk through the week
And whiskey on a Sunday"