Dax for Current Week & Previous Week

You could add a variable to calculate the most recent monday (relative to the MAX date), and then calculate your measure between the max date and that Monday date.

Weekly =
VAR _selectedDate =
    MAX ( Table1[DayDate] )
VAR _endOfWeekDate =
    _selectedDate + 7
        - WEEKDAY ( _selectedDate, 2 )
VAR lastMonday =
    CALCULATE (
        MAX ( Table1[DayDate] ),
        FILTER (
            ALL ( Table1[DayDate] ),
            Table1[DayDate] <= _selecteddate
                && WEEKDAY ( Table1[DayDate] ) = 2
        )
    )
RETURN
    CALCULATE (
        SUM ( Table1[TH] ),
        FILTER (
            ALL ( Table1[DayDate] ),
            Table1[DayDate] <= _endOfWeekDate
                && Table1[DayDate] >= lastMonday
        )
    )

Pat