DAX: Cumulative Comparison (Count row from table visual)

zeroblack7 , For WOW , please refer to my blogs

Power BI — Week on Week and WTD
https://medium.com/@amitchandak.1978/power-bi-wtd-questions-time-intelligence-4-5-98c30fab69d3
https://community.powerbi.com/t5/Community-Blog/Week-Is-Not-So-Weak-WTD-Last-WTD-and-This-Week-vs-Last-Week/ba-p/1051123

or video : https://www.youtube.com/watch?v=pnAesWxYgJ8

You need to create few columns in the date table :

Week Start date = 'Date'[Date]+-1*WEEKDAY('Date'[Date],2)+1
Week End date = 'Date'[Date]+ 7-1*WEEKDAY('Date'[Date],2)
Week Rank = RANKX(all('Date'),'Date'[Week Start date],,ASC,Dense)

And then you can have measures like

This Week = CALCULATE(sum('order'[Qty]), FILTER(ALL('Date'),'Date'[Week Rank]=max('Date'[Week Rank])))
Last Week = CALCULATE(sum('order'[Qty]), FILTER(ALL('Date'),'Date'[Week Rank]=max('Date'[Week Rank])-1))
Last year Week= CALCULATE(sum('order'[Qty]), FILTER(ALL('Date'),'Date'[Week Rank]=(max('Date'[Week Rank]) -52)))
Last 8 weeks = CALCULATE(sum('order'[Qty]), FILTER(ALL('Date'),'Date'[Week Rank]>=max('Date'[Week Rank])-8 && 'Date'[Week Rank]<=max('Date'[Week Rank])))
last two weeks = CALCULATE(sum('order'[Qty]), FILTER(ALL('Date'),'Date'[Week Rank]<=max('Date'[Week Rank])-1
&& 'Date'[Week Rank]>=max('Date'[Week Rank])-3))

Hi zeroblack7.  Something like this???

You can download the modified pbix here.  

Modified the date table to include year and week number (along with a couple other things).  From there the logic is essentially find the most recently completed week and then determine sales for it and the previously completed week.  Subtract and calculate % change.  Measures are below.

Total Sales Previously Completed Week = 
    VAR CalendarSummary =
        GROUPBY(
            FILTER(
                ALL(dDate),
                dDate[Date] <= SELECTEDVALUE(dDate[Date])
            ),
            dDate[Year],
            dDate[WeekNumber],
            "DayCount",
            COUNTX(
                CURRENTGROUP(),
                dDate[Date]
            ),
            "WeekEndingDate",
            MAXX(
                CURRENTGROUP(),
                dDate[Date]
            )
        )
    VAR PreviousWeekEnding =
        MAXX(
            FILTER(
                CalendarSummary,
                [DayCount] = 7
            ),
            [WeekEndingDate]
        )
    RETURN

    CALCULATE(
        [TotalSalesAmount],
        DATESINPERIOD(
            dDate[Date],
            PreviousWeekEnding - 7,
            -7,
            DAY
        )
    )

Total Sales Most Recent Completed Week = 
    VAR CalendarSummary =
        GROUPBY(
            FILTER(
                ALL(dDate),
                dDate[Date] <= SELECTEDVALUE(dDate[Date])
            ),
            dDate[Year],
            dDate[WeekNumber],
            "DayCount",
            COUNTX(
                CURRENTGROUP(),
                dDate[Date]
            ),
            "WeekEndingDate",
            MAXX(
                CURRENTGROUP(),
                dDate[Date]
            )
        )
    VAR PreviousWeekEnding =
        MAXX(
            FILTER(
                CalendarSummary,
                [DayCount] = 7
            ),
            [WeekEndingDate]
        )
    RETURN

    CALCULATE(
        [TotalSalesAmount],
        DATESINPERIOD(
            dDate[Date],
            PreviousWeekEnding,
            -7,
            DAY
        )
    )

Weekly Variance = [Total Sales Most Recent Completed Week] - [Total Sales Previously Completed Week]

Weekly % Variance = 
    DIVIDE(
        [Weekly Variance],
        [Total Sales Previously Completed Week],
        BLANK()
    )

Hope this helps!