Edit current user's crontab:

crontab -e

List user's crontab:

crontab -l

Rule:

0 0 * * * "command"

The first five fields shall be integer patterns that specify the following:

1. Minute [0,59]
2. Hour [0,23]
3. Day of the month [1,31]
4. Month of the year [1,12]
5. Day of the week ([0,6] with 0=Sunday)

Symbol:

* (asterisk) means every xxx, for example, * * * * * means every minutes;

, (comma) means or, such as 1,22 means every 1/22 [classifier];

- (hyphen) means to, such as 1-10 means from 1 to 10 [classifier];

*/ means every x [classifier], */3 [Hour field] means every 3 hours.