Can you assign specific working hours for each employee? Some work Saturdays, others Sunday? Some work 4 days per week, others 5? Some work 16 hours and other 37.5 and you can input what their hours are for each day?
When booking leave, would the system know, for instance, that Andrew works Weekdays, and he books a holiday from Thursday and returns Tuesday he has taken 3 days holiday? ie: He does not work Saturday or Sunday anyway.