Many companies end their accounting periods on the last day of each month, but some companies, usually in manufacturing (such as water heaters) close on the Saturday closest to the end of the month. Thus they keep comparisons consistent: 4 weeks per month, 13 weeks per quarter, 52 weeks per year. However, that amounts to only 364 days for a 52 week year. Because of the extra day each year, and then another for a leap year, every 3 or 4 years, they end up with 53 weeks in the year in order to still close on the Saturday NEAREST the end of the month, which, of course, messes up their comparisons anyway.
So, does 2011 have 53 weeks? Possibly and probably since 2008 was a leap year.
Usually it doesn’t matter because many businesses are closed that last week of the year, so no one ever knows that it was week 53 instead of week 52.