BUCKET([Date], 1, "week", "dd MMM yyyy") This formula can be used for output format like: 26 Dec 2011 to 02 Jan 2012
If the default week start/end dates do not fit with your company/industry - please see the Regional date settings, as these can be customised. Settings>Advanced file settings>Regional
Using the same principle, you could create a new field [Week commencing], adding one more step.