The Holiday calendar not only auto-populates the timesheet (if auto-population is enabled), but also makes sure additional hours are not added to a timesheet on a Holiday. If a user wants to add another Time Off on a Holiday, follow the steps given below.
Edit the user's profile and set the holiday calendar to None.
Got to Historical Time Off and delete the Time Off booking for the Holiday.
Add/Book the Sick Time Off for the user.
Edit the user's profile and set the holiday calendar back to the user's original calendar.