Cron

List crontab
crontab -l

Edit crontab
crontab -e

Syntax

*     *     *   *    *        command to be executed
-     -     -   -    -
|     |     |   |    |
|     |     |   |    +----- day of week (0 - 6) (Sunday=0)
|     |     |   +------- month (1 - 12)
|     |     +--------- day of        month (1 - 31)
|     +----------- hour (0 - 23)
+------------- min (0 - 59)

Enable cron in linux
Check the files cron.allow and cron.deny. Add users to cron.allow to allow to execute cron.