Please guide me how to schedule the cron job to run on every Saturday at 6am with the interval of 21 days.
You might be better off with the at command, rescheduling itself with each run?
0 6 * * 6 lr=./lastrun; [ "`find $lr -mtime -21`" ] || { touch $lr; /path/to/real/job; }
Assuming that . (the home directory) is writable.
The find will report the file if less than 21 days old, then nothing is done.
Otherwise the file is refreshed and the job is done.