The exam key periods are published at the start of the year. It is your responsibility to make yourself aware of the exams you need to take during the academic year and in which month they take place.
If an exam is scheduled to take place during one of the key periods, you must make yourself available for the duration of the exam period.
However, you may be able apply to sit your exams at a different location to your usual place of study (Offsite Exams).