On August 22nd, I commented on a thread about the process of choosing a successor to Senator Ted Kennedy, and I said that the point was moot because "Sen. Kennedy is very much alive at the moment." Sadly, that moment has now passed.
The constitution (2nd paragraph of Amendment XVII) says:
When vacancies happen in the representation of any State in the Senate, the executive authority of such State shall issue writs of election to fill such vacancies: Provided, That the legislature of any State may empower the executive thereof to make temporary appointments until the people fill the vacancies by election as the legislature may direct.
So, according to Amendment XVII, Massachusetts Governor Patrick must schedule a new election, although there is no set timeline for when he has to do so. And, the legislature can— but does not have to— make a law allowing the Governor to appoint a replacement.
Currently, Massachusetts is one of the few states where the governor is not empowered to make a temporary Senate appointment to fill the seat till the next election. A few days before he died, Sen. Kennedy reportedly sent the Massachusetts Speaker of the House and State Senate President a letter urging them to change the law. (Both houses of the state legislature are heavily Democratic: 143-16 in the House and 35-5 in the Senate.)
The current law requires the Governor to schedule an election within five months of Kennedy's passing, i.e., before January 25, 2010.
The next regularly scheduled statewide election is not till September 2010 (primary election day.) Municipalities who are organized as Cities have a uniform Municipal Election Day: November 3, 2009. (There are several municipalities, e.g., the Town of Newton, which call themselves Towns but actually operate as Cities. And towns which are organized as Towns can have their town meetings and elections whenever they want: most have them in the late winter or early spring.)
So, November 3rd would be the logical date for a special election. But logic doesn't always rule in politics, especially not Massachusetts politics.
Kennedy was really only seriously challenged once in his long career, by Mitt Romney in 1994. The GOP put up joke candidates in 2000 & 2006, just to fill the spot on the ballot. But the Republicans have had some success at the statewide level in the Bay State: they held the governor's office from 1991 to 2007. If Romney or someone of his national stature runs, it could be an interesting race.
(The author is from New Hampshire, but he worked on Beacon Hill for for Mike Dukakis for a few months in 1989.)