From:Steve Adams
Date:27-Apr-2001 15:55
Subject:   Running a cron job fortnightly

The command just needs to check whether it is an odd or even week as follows ... 

#----+-----+-----+-----+-----+-------------------------------------------------
#min |hour |day  |month|day  |command
#    |     |of mn|     |of wk|
#----+-----+-----+-----+-----+-------------------------------------------------
03    04    *     *     4     expr `date +%W` % 2 >/dev/null || run_my_script
That will run the script on even weeks only. If you want odd weeks use 
    expr `date +%W` % 2 >/dev/null && run_my_script || true
to prevent cron from complaining about the non-zero return code. Also, watch out for the expr syntax for the mod/remainder function. On some older versions of expr you need to use " instead of %.

I would like to delete some files every OTHER Thursday. I know how to do it every Thursday but I need to skip a Thursday before running the script again. I would like to do this with cron. How?