You probably declared "weekday" as a variable somewhere else, which would cause the error.  See picture below.

I see where I missed up at... I missed Dim the wrong variable name. Thanks Kevin...

But now I ran into another Error...

"weekday" is still in lower case, so it seems Access isn't recognizing the function.  Go to Tools -> References to see if anything is amiss.  Run the Weekday function in the immediate window and see if it works there.  Use Weekday() in another module to see if it works there.  Use it in a new database to see if it works.  This is to find out if the problem lies in your current module, current database, or any database.

Hey Kevin, I figured it out... But I ran into this ERROR...

Wrong number of arguments in your DateSerial() function:

Syntax: DateSerial(Year, Month, Day)

Okay, Kevin. I may have to create a Function the handle the "WeekdayName" and  "onTheInterval". Which "WeekdayName" the Days of the Weekday and "onTheInterval" is 'First, Second, Third, Fourth, or Last' day or week of Month.