A Commencement Order is a form of Statutory Instrument which is designed to bring into force the whole or part of an Act of Parliament at a date later than the date of the Royal Assent. If there is no Commencement Order, the Act will come into force from midnight at the start of the day of the Royal Assent.
Until 1793 an Act of Parliament came into force at the start of the new session. After 1793 the Act came into force at midnight on the day specified.
Further information on this subject can be found from the following link.
Brief Guide: Delegated legislation ( PDF 350 KB)