Listing crontab list :
crontab -l

Crontab edit:
crontab -e

Structure:
.---------------- minute (0 - 59)
| .------------- hour (0 - 23)
| | .---------- day of month (1 - 31)
| | | .------- month (1 - 12) OR jan,feb,mar,apr ...
| | | | .---- day of week (0 - 7) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
| | | | |
* * * * * command to be executed

Everyday at 00:04:
4 0 * * * command

Everday at 00:09:
9 0 * * * command

Every monday at 03:03:
3 3 * * 1 command

Every fifteen minutes:
*/15 * * * * command

Every 5 minutes:
*/5 * * * * command