Use a date math trick in a shell script that I named 'last-day-of-month.sh'. The script will exit with a 'success' (zero) status only if today is the last day of the month. On any other day, it will exit with an 'error' status (one). 

#!/bin/bash

TODAY=`/bin/date +%d`
TOMORROW=`/bin/date +%d -d "1 day"`

# See if tomorrow's day is less than today's
if [ $TOMORROW -lt $TODAY ]; then
        exit 0
fi

exit 1
Which can then be used within a crontab file as follows: 
12 0 * * * /usr/local/bin/last-day-of-month.sh && /run/my/cron/job.sh