If you want a job to run on the last sunday of every month, you can use the following syntax from within cron:
18 * * * 0 [`date "+%d"` -gt 24] && /path/to/script
i.e. on sundays at 18:00 check if the day of the month is greater than 24 - if so run the job (if 23 is
specified the job will run on the last 2 sundays of the month)
NOTE: There back-ticks around the date command, not single quotes.