An employer can use your employment contract or an annual leave policy to set out when in the year you must take or avoid taking holiday, and how many days you can take in one stretch.
For example, your contract might stipulate that you save holiday for a shutdown over Christmas, or that you take leave during your contractual notice period.
Employers will normally ask you to give notice of when you intend to take annual leave. You must give notice of at least twice the number of days you want to take. This may be varied by your contract or a collective agreement.
If your employer refuses to grant a request for holiday, they must give you notice of at least as many days as you have requested to take. For example, if you wanted to take two weeks' holiday, the employer must say no at least two weeks before you wanted your leave to start.