The Governor has the power "to call together the General Assembly, when necessary, before the day to which they shall stand adjourned." (Chapter II, Section 20, Vermont Constitution). That language has remained unchanged from the original 1777 Constitution (in 1777 it can be found in Chapter II, Section XVIII; in the 1786 Constitution it is in Chapter II, Section XI; and it was in Chapter II, Section 11 when the 1793 Constitution was first ratified).
A governor calls for a special session through a proclamation setting the date and a brief statement on the necessity for the session. Usually the governor will address the special session, elaborating on the reason for calling the session.
There is no definition of what constitutes “when necessary” and special sessions, also known as extraordinary or extra sessions, have been called for a variety of reasons. The first special session was in 1857, called in response to the statehouse fire. There have been, as of June 2009, 25 special sessions. The most common reasons for calling special sessions have been fiscal problems, federal legislation, war, and disaster (fire or flood, for example).
Special sessions are not a continuation of the regular session, though since 2005 the distinctions between special and regular sessions have begun to blur. Usually a special session must deal with new matters, excluding bills introduced in the regular or adjourned sessions. The senate in 2005 and both chambers in 2009 added a “not withstanding” clause to their rules allowing bills from the regular session to be taken up.
Bills introduced but not enacted during the special session die and do not become pending when the legislature reconvenes. Special session bills and acts are numbered from one forward (House Bill No. 1, Senate Bill No. 1, Act No. 1, etc). There is no set duration for a special session; in 1966 the special session lasted 66 days. Most special sessions run for less than a week; often they are completed within a couple of days.
Special sessions are different than veto sessions. A veto session occurs if the legislature, as part of its adjournment resolution, includes language allowing it to reconvene to address any vetoes. Veto session language started in 1995 but has not been uniformly added as part of adjournment. As of 2013, there have been six veto sessions.
Additional information on special sessions is available on the Vermont State Legislature's web site.