Hi bbqq021 ，

You could try the following steps:

Step1，use the following dax ,to create data table:

Dates = CALENDAR("2016/1/1","2017/12/31")

Step2, import holiday in 2016 and 2017,refer:

Creating A Calendar Table For Holidays In Power BI

https://blog.enterprisedna.co/creating-a-calendar-table-for-holidays-in-power-bi/ 

And import web link :

https://www.calendarpedia.com/holidays/federal-holidays-2016.html

https://www.calendarpedia.com/holidays/federal-holidays-2017.html 

And you will see the below:

Step 3, base on dates,and holiday table ,to create a new table ,use the following dax(when weekend and holiday ,the column return false)

date1 = 
GENERATE (
    CALENDARAUTO (),
    VAR yyyy =
        YEAR ( [Date] )
    VAR mmmm =
        MONTH ( [Date] )
    RETURN
        ROW ("Date1",FORMAT ( [Date], "YYYY-MM-DD" ),
            "Year", yyyy,
            "Month", FORMAT ( [Date], "mmmm" ),
            "Month2", mmmm,
            "Week1", FORMAT ( [Date], "dddd" ),
            "Week2", WEEKDAY ( [Date] ),
            "weekday", NOT WEEKDAY ( [Date] ) IN { 1, 7 } && not [Date] in VALUES('2016holiday'[Date])&& not [Date] in VALUES('2017holiday'[Date])
        )
)

Step 4，use the dax to get the day except weekend and holiday，and then get average:

difforder1 = 
VAR day1 =
CALCULATE (
COUNTROWS ( 'date1' ),
DATESBETWEEN ( date1[Date], Original[Start Date], Original[End Date]),
date1[weekday] <> TRUE,
ALL ( Original )
)
VAR day2 =
IF ( day1 = BLANK (), 0, day1 )
VAR day3 =
DATEDIFF ( Original[Start Date], Original[End Date], DAY ) - day2
RETURN
day3

AVERAGE = Original[Resource]/Original[difforder1]

Final you will get the below:

Wish it is helpful for you!

Best Regards

Lucien