Well that's probably more determined more by luck now than anything.  It is looking like either sometime on the 29th (GMT) or possibly late in the day (again, GMT) on the 28th.   

So it is getting pretty safe to say that if you were to attend an all-day "halving day" party on Wednesday, Nov 28th, 2012 in the U.S., you would be present to raise your glass for a toast at the point that block 210,000 is solved.