Each centre taking our exams is given specific Key Times based on their country and location, when all candidates taking timetabled exams must be either in an exam or under Full Centre Supervision. Please see the relevant Cambridge Handbook for further information on Key Times and Full Centre Supervision.

http://www.cambridgeinternational.org/cambridge-for/exams-officers/cambridge-exams-officers-guide/phase-1-preparation/timetabling-exams/key-times/