Define range – You can define range using the hyphen like: 1-10 or 20-30 or sun-fri or feb-apr.Multiple values – Use the command (,) to define multiple values like 2,4,8 or sun,fri or jan,oct,dec etc.Now, the below statements will describe how to define multiple values or ranges. You can also use the first three alphabets of days like sun, mon, tue, wed, etc.
Where 0 and 7 for Sunday, 1 for Monday, 2 for Tuesday, and so on.
Day_of_the_Week – This can be the value between 0-7. You can also define this value with the first three alphabets of the month like jan, feb, mar, apr etc. Month_of_the_year – This can be between 1-12. For the months having fewer days will ignore remaining part 
Day_of_the_month – This value can between 1-31. Hour – A hour value can be between 0-23. Minute – A minute value can be between 0-59. For example, an asterisk in the fourth field indicates that the job should be run every week. Wildcards can also be used in crontab files. The first field specifies the minute (0-59) when the job will be run, the second field specifies the hour (0-23), the third field specifies the day of the month (1-31), and so on. The format of a crontab file is very simple: each line contains six fields, separated by spaces. If you do not have crontab installed on your system refer to article Install Crontab in CentOS/RHEL. You can also use crontab for the tasks to run once in the future only, but for any tasks to run once we recommend using Linux at command. In this tutorial, you will learn to use crontab with 20 useful examples for scheduling jobs. Crontab syntax is mostly the same across all Linux distributions and servers, so once you get the hang of it you’ll be scheduling tasks in no time! Creating a cron job is not difficult, but it can become tricky when dealing with multiple users and environments. The crontab command is used to edit and manage cron jobs. Cron is an essential part of any system, as it automates various scheduled tasks.
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