birthnight

from The Collaborative International Dictionary of English v.0.48

Birthnight \Birth"night`\, n.
   The night in which a person is born; the anniversary of that
   night in succeeding years.
   [1913 Webster]

         The angelic song in Bethlehem field,
         On thy birthnight, that sung thee Savior born.
                                                  --Milton.
   [1913 Webster]