Immediately following each regular session of the General Assembly, the Commission shall arrange for the codification and incorporation into the Code of Virginia of all general and permanent statutes enacted at such regular sessions and at all special sessions that have occurred between that regular session and the immediately preceding regular session. The statutes may be incorporated by supplements to each volume, replacement of any volume or volumes, or a combination thereof. Unless prevented by unusual circumstances, this work and the distribution of each supplement and replacement volume shall be completed prior to the date when the statutes contained in each such supplement and replacement volume become effective.