• The cryptic property is the date of statehood (the date of admission to the union).
  http://en.wikipedia.org/wiki/List_of_U.S._states_by_date_of_admission_to_the_Union

• The list has the statehood dates ordered upside down: the younger states come at the beginning, the later states come at the end

• The first missing state is Nebraska (admission date = March 1, 1867).
  The second missing state is Mississippi (admission date = December 10, 1817).

• The cryptic property is:

the date of statehood (the date of admission to the union).
http://en.wikipedia.org/wiki/List_of_U.S._states_by_date_of_admission_to_the_Union
The list has the statehood dates ordered upside down: the younger states come at the beginning, the later states come at the end

• The first missing state is:

Nebraska (admission date = March 1, 1867).

• The second missing state is:

Mississippi (admission date = December 10, 1817).