*     *   *   *    *  command to be executed
-     -    -    -    -
|     |     |     |     |
|     |     |     |     +----- day of week (0 - 6) (Sunday=0)
|     |     |     +------- month (1 - 12)
|     |     +--------- day of month (1 - 31)
|     +----------- hour (0 - 23)
+------------- min (0 - 59)


With the 5th field, you cannot set 'the 26th friday'. But:

0 12 * * 5 (( `date +%W` == 26 ))&&your_command

will check if week number is 26 each friday at 12:00 AND if true, launch your_command...