Mo, Wiktionary gives a total of 4 citations:

As an adverb

1) 1535, Myles Coverdale, The Byble, that is, the Holy Scrypture of the Olde and New Teſtament, faythfully tranſlated into Englyſhe[1], Tobit 8:4, page D.iiij:
Thē ſpake Tobias unto the virgin, and ſayde: Up Sara, let us make oure prayer unto God to daye, tomorow, and ouermorow: for theſe thre nightes wil we reconcyle oure ſelues with God: and whan the thirde holy night is paſt, we ſhall ioyne together in ye deutye of mariage.
Then spake Tobias unto the virgin, and said: Up Sara, let us make our prayer unto God today, tomorrow, and overmorrow: for these three nights will we reconcile ourselves with God, and when the third holy night is past, we shall join together in the duty of marriage.

2) 1925, Parliamentary Debates: Official Report[2], volume 188, H.M. Stationery Off., page iv:
We can go not overmorrow, but on Thursday.

3) 1969, James Klugman quoting Bucharin, History of the Communist Party of Great Britain: The General Strike, 1925-1927[3], volume 2, London: Lawrence & Wishart, page 73:
Sinowjeff and myself go to Caucasus overmorrow.

As a noun

4) 1898, Johann Wolfgang von Goethe, The first part of the tragedy of Faust, Longmans, Green and Co., page 197:
My prescient limbs already borrow
From rare Walpurgis-night a glow :
It comes round on the overmorrow [transl. übermorgen] —
Then why we are awake we know.

Clearly, a word that can go 350 years between citations is not what one would call common! Seems to have been resurrected (briefly) in the 1920s.