FirstDayOfWeekUS() -

Function reference
Get the first day of the week as a date data type. (US standard)

FirstDayOfWeekUS( year , week )

Argument	Description
year	a numeric value for the year
week	a numeric value for the week number

Data type: Date

Example:
Expression returns 2013-10-20 which is the date of the first day in week 43. (US standard)
FirstDayOfWeekUS( 2013 , 43 )