It turns out that it's due to a quirk in the way TABOR was written into the Colorado Constitution. Article X, Section 20 specifies that votes on taxes, spending and debt occur on the "first Tuesday of November in odd-numbered years."
Other elections this month, however, including governors' races in New Jersey and Virginia, follow the usual calendar: the first Tuesday after the first Monday in November. That's a tradition that goes back to the days when people might have to travel for a whole day, perhaps on horseback, to get to the polls.
Monday was too close to the Christian Sabbath--hence Tuesday. The second-after-the-first bit had two rationales. First, a Nov. 1 election was a no-no because it is All Saints' Day. Plus, the first of the month was traditionally the day that merchants settled their books.
I'm not sure if the TABOR language was an oversight or deliberate. Apparently, this is the first time the current discrepancy has occurred. As the Rocky Mountain News reports, voters in some Colorado cities will have to vote twice: once today on the TABOR issue, and again on Nov. 8 for other local issues.
