There are two places which you can set that particular day as compulsory working day:
1) Employee Custom Day Entry
In the employee custom day entry screen, you will need to tick on the 'Compulsory' checkbox to indicate that it is a compulsory working day for the dates that has been selected in the Custom Day Entry.
2) GPS Duty Roster
In the GPS Duty Roster tab, you may tick on the day that you would like to set as compulsory working day, and tick on the 'Compulsory Working Day' checkbox to indicate as compulsory working day.