Nobody is entitled to have any particular day off. One of the many problems with this country now is that no day is considered special. If you decide that they need to work it then you can demand they do - and you dont have to pay them overtime or extra money unless it is written in their contract.
Obviously, you can tell them all you like - the individual has the right to leave if they dont like your employment policies. As long as you are fair to everyone, go for it, but I would suspect unless you are willing to bend a little you will personally be very busy that day - as all your staff will ring in sick !!!