Taken from man -s5 crontab:
<p>Commands are executed by cron when the minute, hour, and month of year 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 (see "Note" below). Note that this means that non-existent times, such as "missing hours" during daylight savings conversion, will never match, causing jobs scheduled during the "missing times" not to be run. Similarly, times that occur more than once (again, during daylight savings conversion) will cause matching jobs to be run twice.</p>

<p>cron examines cron entries once every minute.</p>

<p>The time and date fields are:</p>
<pre>
      field          allowed values
      -----          --------------
      minute         0-59
      hour           0-23
      day of month   1-31
      month          1-12 (or names, see below)
      day of week    0-7 (0 or 7 is Sun, or use names)
</pre>
<p>A field may be an asterisk (*), which always stands for "first-last".</p>

<p>Ranges of numbers are allowed. Ranges are two numbers separated with a hyphen. The specified range is inclusive. For example, 8-11 for an "hours" entry specifies execution at hours 8, 9, 10 and 11.</p>

<p>Lists are allowed. A list is a set of numbers (or ranges) separated by commas. Examples: "1,2,5,9", "0-4,8-12".</p>

<p>Step values can be used in conjunction with ranges. Following a range with "<number>" specifies skips of the number's value through the range. For example, "0-23/2" can be used in the hours field to specify command execution every other hour (the alternative in the V7 standard is "0,2,4,6,8,10,12,14,16,18,20,22"). Steps are also permitted after an asterisk, so if you want to say "every two hours", just use "*/2".</p>

<p>Names can also be used for the "month" and "day of week" fields. Use the first three letters of the particular day or month (case doesn't matter). Ranges or lists of names are not allowed.<p>

<p>The "sixth" field (the rest of the line) specifies the command to be run. The entire command portion of the line, up to a newline or % character, will be executed by /bin/sh or by the shell specified in the SHELL variable of the cronfile. Percent-signs (%) in the command, unless escaped with backslash (\), will be changed into newline characters, and all data after the first % will be sent to the command as standard input.</p>

<p>Note: The day of a command's execution can be specified by two fields: day of month, and day of week. If both fields are restricted (ie, aren't *), the command will be run when either field matches the current time. For example, "30 4 1,15 * 5" would cause a command to be run at 4:30 am on the 1st and 15th of each month, plus every Friday.</p>
