The requirement for entry into Turkey with a Schengen visa is that your Schengen visa should still be valid (by date) by the time you enter Turkey.

So if your visa is valid on the day of entry, then you can enter Turkey even though it expires the next day! 

BUT if your visa is already expired upon arrival, then its not valid.


NOTE  - Previously used or unused visas are accepted as long as their validity date covers your entry date to Turkey.