Total Quantity on Weekdays

 Total Quantity (Weekday) =

CALCULATE(

[Total Quantity],

FILTER(

'Internet Sales',

RELATED('Date'[DayNumberOfWeek]) > 1 &&

RELATED('Date'[DayNumberOfWeek]) < 7))

-- filter on the one side of the relationship

Total Quantity (Weekday) =

CALCULATE(

[Total Quantity],

FILTER(

'Date',

AND('Date'[DayNumberOfWeek] > 1,

'Date'[DayNumberOfWeek] < 7)))