Averages per Weekday

To report on things that are not there you need to use disconnected tables and/or crossjoins

Add a disconnected calendar table to your data model and then use it in the visual.  Your measure can then do the rest.

This solution seems to work with the sample data and an unconnected calendar:

AverageSalesIncludingNoSales = 
VAR CurrentWeekday = SELECTEDVALUE(UnconnectedCalendar[Weekday])  // Get the current weekday from the visual context
VAR TotalSales = CALCULATE(
    SUM(Sales[Sales]),
    FILTER(Sales, WEEKDAY(Sales[Date], 2) = CurrentWeekday)  // Filter sales for the current weekday
)
VAR TotalWeekdays = COUNTROWS(
    FILTER(
        UnconnectedCalendar,
        UnconnectedCalendar[Weekday] = CurrentWeekday  // Count all instances of the current weekday
    )
)
RETURN
IF(
    TotalWeekdays = 0,
    0,
    DIVIDE(TotalSales, TotalWeekdays, 0)  // Use 0 as an alternate result for division by zero
)