If you're repeatedly told that you can't take leave days during a period of time when you're entitled to take leave under your contract, you should raise this with your manager or HR representative to find out what the reasons are and whether they are justified. If the issue isn't resolved satisfactorily, you can raise a formal grievance with your employer. You should find advice on how to do this in your staff handbook or contract. You may also decide to seek legal advice, either independently or as a member of a Trade Union. It's worth noting, however, that if there's a genuine business reason why you can't take days off at a certain time then that's likely to be an adequate response from your employer. Even so, you may want to remind your employer that they're legally required to allow you to take your statutory minimum holiday within the holiday year. It may also be a breach of your contract of employment if they don't allow you to take your full contractual entitlement.