There are currently three vacancies within the Most Noble Order of the Garter. The royal website states that
If there are vacancies in the Order, appointments are announced on St George's Day (23 April).Former prime ministers are invariably appointed to the Order, Sir John Major being the last prime ministerial appointment.
Two former prime ministers have yet to be appointed, though membership of the Order is within the personal gift of the sovereign.
There has been speculation that Prince Henry of Wales ~ Prince Harry ~ shall be appointed to the Order shortly.