Really difficult. I want to say May 2012 or June 2012, because if you remember, Mario Party 8 came out late May 2007 in North America. It could be February 2012 or March 2012, but I still doubt those dates.
And December 2011 and January 2012? Hell no. We would have had more information about Mario Party 9 by now if it were to be released in December 2011 or January 2012. Also, with Fortune Street coming up in December 2011 and January 2012, that makes it unlikely for Mario Party 9 to be released and steal Fortune Street's spot. So I'm saying late Spring or early Summer as a safe bet.
I still strongly agree with Timmy's point about Mario Party 9 if it were to be released in December 2011 or January 2012.