Se você pesquisar por dias úteis na referência do desenvolvedor do Access, ele fornecerá o código que fará esse cálculo para você
[Reproduzido para a posteridade]
Function Work_Days(BegDate As Variant, EndDate As Variant) As Integer
Dim WholeWeeks As Variant
Dim DateCnt As Variant
Dim EndDays As Integer
On Error GoTo Err_Work_Days
BegDate = DateValue(BegDate)
EndDate = DateValue(EndDate)
WholeWeeks = DateDiff("w", BegDate, EndDate)
DateCnt = DateAdd("ww", WholeWeeks, BegDate)
EndDays = 0
Do While DateCnt <= EndDate
If Format(DateCnt, "ddd") <> "Sun" And Format(DateCnt, "ddd") <> "Sat" Then
EndDays = EndDays + 1
End If
DateCnt = DateAdd("d", 1, DateCnt)
Loop
Work_Days = WholeWeeks * 5 + EndDays
Exit Function
Err_Work_Days:
' If either BegDate or EndDate is Null, return a zero
' to indicate that no workdays passed between the two dates.
If Err.Number = 94 Then
Work_Days = 0
Exit Function
Else
' If some other error occurs, provide a message.
MsgBox "Error " & Err.Number & ": " & Err.Description
End If
End Function