Working Days are shown in the In the Salary Slip. Based on your preference, it may include holidays of the month or it may not. You can define your preference for the Working Days calculation in 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.