If specific days appear "blocked" or shaded on an employee's leave calendar but aren't set up under Configuration → Holidays, the usual cause is a partial-day leave application. When an employee applies for a part-day leave (for example a 4-hour duration), the day is only partly consumed and can display differently from a full working day. Check the employee's leave requests for that period — a partial-day entry, not a holiday, is almost always what's showing.