# m h  dom mon dow   command

# MIN HOUR MDAY MON DOW COMMAND

Code:
MIN = Minute 0-60 HOUR = Hour [24-hour clock] 0-23
MDAY = Day of Month 1-31 
MON = Month 1-12 OR jan,feb,mar,apr ... 
DOW = Day of Week 0-6 OR sun,mon,tue,wed,thu,fri,sat COMMAND = Command to be run Any valid command-line

@reboot Run once, at startup. None

@yearly Run once a year 0 0 1 1 *

@annually (same as @yearly) 0 0 1 1 *

@monthly Run once a month 0 0 1 * *

@weekly Run once a week 0 0 * * 0

@daily Run once a day 0 0 * * *

@midnight (same as @daily) 0 0 * * *

@hourly Run once an hour 0 * * * *

Example: 
sudo -i
crontab -e

#reindex locate db daily
0 0 * * * /usr/bin/updatedb

ubuntu cron reference

Bash