You need to remove the incorrect public holiday first in the calendar setup, followed by selecting the correct public holiday. Please follow the steps below to do this;
Remove the wrong date
Go to Core, Setup, Calendar Setup.
Select the calendar ID that you are using, for example, 5.5Days Week Cal.
Click the three-dotted button.
Click Holidays Country ID and click the three-dotted button.
Select the date, click Delete, and click OK.
Close the holidays setup and calendar setup screen.
Right-click the date and change the from P to F (Full workday) for Work Calendar and Leave Calendar.
Add the correct date
Go to Core, Setup, Calendar Setup.
Select the calendar ID that you are using, for example, 5.5Days Week Cal.
Click the three-dotted button, click Holidays Country ID, and click the three-dotted button.
Click New, select Holiday ID and add the Description e.g., Muhammad's Birthday
Set the date, click Save, and select Yes.