For Enterprise, Unlimited, Professional, and Developer Editions, the week is defined by the Locale drop-down list in your personal information settings at Setup | My Personal Information | Personal Information. For Group and Personal Editions, the week is defined by the Locale setting in the company profile at Setup | Company Profile | Company Information. For example, when the Locale is US English, a week runs Sunday to Saturday, whereas with UK English, a week spans Monday to Sunday.
According to the help files:
For Enterprise, Unlimited, Professional, and Developer Editions, the week is defined by the Locale drop-down list in your personal information settings at Setup | My Personal Information | Personal Information. For Group and Personal Editions, the week is defined by the Locale setting in the company profile at Setup | Company Profile | Company Information. For example, when the Locale is US English, a week runs Sunday to Saturday, whereas with UK English, a week spans Monday to Sunday.
http://www.salesforce.com/us/developer/docs/api/index_CSH.htm#sforce_api_calls_soql_select_dateformats.htm
Good luck!