Preview:
Sales-PW = 

VAR CurrentWeek =
    SELECTEDVALUE( 'Calendar'[WeekNo] )

VAR CurrentYear =
    SELECTEDVALUE( 'Calendar'[Year] )

VAR MaxWeekNo =
    CALCULATE(
        MAX ( 'Calendar'[WeekNo] ), 
            ALL ( 'Calendar' )
            )

RETURN
SUMX(
    FILTER( ALL ( 'Calendar' ),
        IF ( CurrentWeek = 1,
            'Calendar'[WeekNo] = MaxWeekNo && 'Calendar'[Year] = CurrentYear -1,
            'Calendar'[WeekNo] = CurrentWeek -1 && 'Calendar'[Year] = CurrentYear )
        ),
    [Total Sales]
    )
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