Spreadsheet for allowing employees to input vacation leave/show employee denial of leave

I am trying to create a spreadsheet for employees for taking summer vacation
Employees are ranked by seniority.
Only so many employees can be given vacation on a specific date.

I want to have the ability so that all employees can go in and request certain dates. (I've created a calendar).

What I'm trying to figure out if we are allowing 4 people vacation, and 8 people request the day that the people with the lower seniority essentially get bumped. Or that within the cell it shows a different colour or something to show that it is refused.

Don't know if that makes sense. Can provide additional information if you need.