As Chris says, "unless you have a contract....."
Most employers will have some contractual conditions around taking holidays and these will often concentrate on dealing with the issues of running a business such as continuity of cover, where the contactual term will often relate to taking holiday with the employer's agreement. It may also specify that, say two weeks, must be taken together another means of trying to simplify cover.
Frequency of absence may also be a problem in some businesses so you may find a condition which limits the number of single days which may be taken. At the other extrme, in order to balance fairness with cover, the contract may specify that a maximum of, say three weeks, may be taken together, so as to stop somebody grabbing a particular peak time, so others can't take it, with all the emotions that that can arouse!
Another issue is often related to shut downs when the premises are closed, and the contract may specify that holiday is reserved for that. In my experience, having done that, employers often then find that they need people in over the shutdown, for example engineers on maintenance, or accountants at Christmas/ New Year to complete the year end accounts, and have to start making special agreements.
So read your own contract very carefully!!