the plugin module uses Cron Scripts to allow user to specify when the job / task needs to be executed. If the user didn't specify anything at all, the Cron script without values will be shown as '*******' (Seven asterisk).
The user must specify a value for each field. Except for the year field, these fields can contain the following:
- A number in the specified range. To run a command in May, specify 5 in the month field.
- Two numbers separated by a dash to indicate an inclusive range. To run a cron job on Tuesday through Friday, place 2-5 in the weekday field.
- A list of numbers separated by commas. To run a command on the first and last day of January, you would specify 1,31 in the day_of_month field.
- A combination of two numbers separated by a dash to indicate an inclusive range and a list of numbers separated by commas can be used in conjunction. To run a command on the first, tenth to sixteenth and last day of January, you would specify 1,10-16,31 in the day_of_month field. The above two points can also be used in combination.
- An * (asterisk), meaning all allowed values. To run a job every hour, specify an asterisk in the hour field.
Note: Any character preceded by a backslash (including the %) causes that character to be treated literally. The specification of days may be made by two fields (day of the month and day of the week). If you specify both as a list of elements, both are adhered to. For example, the following entry:
Would run command on the first and fifteenth days of each month, as well as every Monday. To specify days by only one field, the other field should contain an * .
Find more from: http://quartz-scheduler.org/documentation/quartz-1.x/tutorials/crontrigger