Working days are displayed in the Salary Slip, and it can be configured based on your preference. This configuration may or may not include holidays of the month, depending on your choice. You can specify your preference for the Working Days calculation in the HR Settings.

HR > Setup > HR Settings

If you want to include holidays in the count of Total Working days, then ensure that in the HR Settings, field Include holidays in Total no. of Working Days is checked and vice versa.

To learn how to define holidays for your company, check Holiday List feature in the HR module.