Cron tab

crontab date/time column definitions:

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

more info on crontab at http://www.adminschoice.com/crontab-quick-reference/