Cron Basics

                                  
cron fields

[minute] [hour] [day of month] [month] [day of week] [cmd]

minute           minute of the hour (0 - 59)
  
hour             hour of the day (0 - 23, 0 being midnight)

day of month     day of month (1 - 31)

month            month of the year (0 - 12)

day of week      day of week (0 - 7, Sunday being 0 or 7) 

cmd              path to command to be run 
 

examples

run every hour at 15 after the hour:
15 * * * * /usr/local/admin/bin/backup.sh

run daily at 4:15am:
15 4 * * * /usr/local/admin/bin/backup.sh

run daily at 3:15pm:
15 15 * * /usr/local/admin/bin/backup.sh

run daily at 4:00am:
0 4 * * * /usr/local/admin/bin/backup.sh

run at 4:15am on the 5th of every month:
15 4 5 * * /usr/local/admin/bin/backup.sh

run hourly on the 15th of April:
0 * 15 4 * /usr/local/admin/bin/backup.sh

run every 15 minutes:
*/15 * * * * /usr/local/admin/bin/backup.sh

run every 15 minutes between 3:00am and 9:00am:
*/15 3-9 * * * /usr/local/admin/bin/backup.sh

comments are allowed in the crontab, are preceded by a # and must be on a line by themselves