Cases per Day Calculation

Add a new column onto your calendar table to flag whether a day is a holiday or not. You can change the existing ADDCOLUMNS to be

Calendar =

VAR Days = CALENDAR(DATE(year(TODAY())-7,1,1),DATE(year(today())+1,12,31))
VAR Holidays = {
        (1, 1),   
        (2, 19),  
        (5, 27),  
        (7, 4),   
        (9, 2),   
        (11, 28), 
        (11, 29), 
        (12, 24), 
        (12, 25)  
    }
Return ADDCOLUMNS(
    Days,
    "Today", TODAY(),
    "Year#", YEAR([Date]),
    "Year", FORMAT(YEAR([Date]),"#"),
    "Month#", VALUE(FORMAT(MONTH([Date]),"#")),
    "Month Number",FORMAT([Date],"MM"),
    "Month", FORMAT([Date],"mmm"),
    "Quarter#",QUARTER([Date]),
    "Quarter","Q"&FORMAT([Date],"Q"),
    "Year Month", FORMAT([Date],"YYYYMM"),
    "Year Quarter", FORMAT(YEAR([Date]),"#") &"Q"&FORMAT([Date],"Q"),
    "Week Number", FORMAT(WEEKNUM([Date]),"00"),
    "Week In Month",FORMAT(WEEKNUM([Date],2) - WEEKNUM(EOMONTH([Date],-1)+1,2)+1,"#"),
    "Is Holiday", ( MONTH( [Date] ), DAY( [Date] ) ) IN Holidays
    )

You can then change the [Shipping Days] measure to be

Shipping Days = VAR MinDate = MIN( Calendar[Date] )
VAR MaxDate = MAX( Calendar[Date] )
VAR Holidays = CALCULATETABLE(
    VALUES( Calendar[Date]),
    Calendar[Is Holiday] = TRUE
)
VAR WorkingDays = NETWORKDAYS( MinDate, MaxDate, 1, Holidays)
RETURN WorkingDays

And the [Case per Ship Day] stays the same.

You should also change the relationship between Calendar and the fact table to be single direction, there's no need for it to be bi-directional.

See the attached PBIX for a working sample.