How to create a measure for subtracting two values from two different dates?

Hi IEPMost ,

Let's take Week Slicer as an example, according to my understand, when you choose Week37 , then calculate 9/11/2020 minus 9/7/2020 and 9/13/2019 minus 9/9/2020 respectively ,right?

2020 Week Measure =
VAR _seleWeek =
    SELECTEDVALUE ( 'Date'[Woche des Jahres] )
VAR _minDay =
    CALCULATE (
        MIN ( 'meter reader 2020'[TST] ),
        FILTER ( ALL ( 'Date' ), 'Date'[Woche des Jahres] = _seleWeek )
    )
VAR _maxDay =
    CALCULATE (
        MAX ( 'meter reader 2020'[TST] ),
        FILTER ( ALL ( 'Date' ), 'Date'[Woche des Jahres] = _seleWeek )
    )
RETURN
    CALCULATE (
        MAX ( 'meter reader 2020'[Kunde017(kWh)] ),
        FILTER ( 'meter reader 2020', 'meter reader 2020'[TST] = _maxDay )
    )
        - CALCULATE (
            MAX ( 'meter reader 2020'[Kunde017(kWh)] ),
            FILTER ( 'meter reader 2020', 'meter reader 2020'[TST] = _minDay )
        )

Or use RANK() 

rank =
RANKX (
    FILTER (
        ALL ( 'Date' ),
        'Date'[Woche des Jahres] = SELECTEDVALUE ( 'Date'[Woche des Jahres] )
            && YEAR ( 'Date'[TST] ) = YEAR ( MAX ( 'Date'[TST] ) )
    ),
    CALCULATE ( MAX ( ( 'Date'[TST] ) ) ),
    ,
    ASC
)

2019 Week Measure =
VAR _maxDay =
    MAXX ( FILTER ( 'Date', YEAR ( 'Date'[TST] ) = 2019 ), [rank] )
RETURN
    CALCULATE (
        SUM ( 'meter reader 2019'[Kunde017(kWh)] ),
        FILTER ( 'Date', [rank] = _maxDay )
    )
        - CALCULATE (
            SUM ( 'meter reader 2019'[Kunde017(kWh)] ),
            FILTER ( 'Date', [rank] = 1 )
        )

My final output looks like this:

Here is the pbix file.

Did I answer your question ? Please mark my reply as solution. Thank you very much.
If not, please upload some insensitive data samples and expected output.

Best Regards,
Eyelyn Qin