Commands are executed by cron when the minute, hour, and month fields match the current time, and at least one of the two day fields (day of month, or day of week) match the current time. A field may be an asterisk (*), which will always match.

FieldAllowed values
minute0-59
hour0-23
day of month1-31
month1-12 or first three letters
day of week0-7 or first three letters
(0 or 7 is Sunday)
Examples
Run every Tuesday at 2:30
30 2 * * tue /path/to/command
Run every 10 minutes
*/10 * * * * /path/to/command
Run every 2 hours, on the half hour
30 */2 * * * /path/to/command
Run every 2 hours, on the half hour, but only on weekdays
30 */2 * * 1-5 /path/to/command
Run at 12:05, 13:05, ..., and 18:05
5 12-18 * * * /path/to/command
Run at 12:05, 14:05, 16:05, and 18:05
5 12-18/2 * * * /path/to/command
Run on the first day of every month, at midnight
0 0 1 * * /path/to/command
Run on the first day of every third month, at midnight
0 0 1 */3 * /path/to/command