Employers are entitled to refuse holiday requests. They might do so, for example, during busy periods or overlaps in holiday requests.
There is no absolute right for employees to take holiday times whenever they like.
You will also have a set amount of notice you will need to give your employer. This can range from a day to a month or so.
Make sure you’re aware of how long you need to give. This should be in your employment contract, company handbook, or intranet.