Week starting date

Hi All,

I want to get the week starting date for the measure which is at item/store intersection having values of effective dates.
e.g) Measure name: effectdate
100457690,7243,20130916
100457702,7243,20130924
100457729,7243,20130914

For the date "20130916", the week start date (sunday date) is "20130915" and for the date "20130924", the week starting date is "20130922".
Like this, i want to get the week start of all the item/loc combinations based on their respective effective date.

I am trying with index function but not able to achieve the expected result.
Please let me know how to achieve the above result.

Thanks,
Tamil