Crontab
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less /etc/crontab	#tells when the cron.x's run

a crontab entry is structured as follows:
<minute> <hour of day> <day of month> <month> <day of week> <user> <program> <arguments>

0 0 1 * * root logrotate /etc/logrotate.conf

You can use the divide-by operator like */2 to indicate every minute (*) divided by 2, thus... every other minute.
* */2 * * * /foo/bar # would indicate every other hour
* * */2 * * /foo/bar # would indicate every other day
0 21 * * * /foo/bar # would indicate every day at 9pm
0 21 * * 1-5 /foo/bar # would indicate every day M-F at 9pm OR
0 21 * * Mon-Fri /foo/bar # same as above

For more examples:
man 5 crontab