How obsolete is the word "overmorrow"?
Solution 1:
It's a bit strange - searching the google books directly for "overmorrow" gives 16 results, see here.
The measure of obsoleteness (and strangeness) is that fact that it is listed in 1913 Webster, but not in 1828 Webster. Also note that many dictionaries do not list it at all (for comparison use the results on onelook, here)
I would say that it is long obsolete word that was never common.