For most purposes in Web analytics, the 31-day period most closely corresponding to October 2012 is not October 2011 but more specifically October 3, 2011 – November 3. 2011.
Because activity on most websites is influenced by day of the week, the one-day offset (or two-day for leap years) more closely aligns the time periods that you’re comparing.
As another example, July and August both contain 31 days, but July 2012 had 5 Sundays while August 2012 had only 4. If Sundays typically see the least traffic to your site, a comparison of several weeks in July and August starting on the same day of the week might provide better insights than a straight month-to-month comparison.