Run cron jobs at the last day of the month

0 8 28-31 * * [ `echo \` cal \ `| awk '(print $ NF)'` - eq 1] & & do - something

Run cron jobs at the last workday of the month

0 8 26-31 * * root [ `echo \` cal | cut-d '' - f2-6 \ `| awk '(print $ NF)'` - eq `e` date +%] & & do - something