It you like to summarise your data into weeks it can be handy to convert a date to a week commencing date. Of course some people start their weeks on Sundays and others on a Monday, so there are two formulae depending on which one you want. This assumes the date is in cell A1.
Week starting on a Sunday
=DATE(YEAR(A1),MONTH(A1),DAY(A1)-WEEKDAY(A1,1)+1)
Week starting on a Monday
=DATE(YEAR(A1),MONTH(A1),DAY(A1)-WEEKDAY(A1,3))