In R6 you can use @BusinessDays to calculate the number of working days between one date and another, excluding non-working days and a list of dates to exclude as well. As there is no equivalent in LotusScript, I wrote a small class to simulate @BusinessDays in LS.
Actually I did not reinvent the wheel, but simply used evaluate in combination with @BusinessDays to achieve the aim.

Here is my code:

Class BusinessDay

	Private holidays As String
	Private nondays As String

	Sub New (strExcludeDays As String,strExcludeDates As String)
		holidays = "[01/01/1899]"
		nondays = "0"
		If strExcludeDays <> "" Then
			nondays = strExcludeDays
		End If

		If strExcludeDates <> "" Then
			holidays = strExcludeDates
		End If
	End Sub

	Public Function GetBusinessDays(dtStart As String,dtEnd As String) As Integer

		Dim bd As Variant
		Dim StartDT As New NotesDateTime(dtStart)
		Dim EndDT As New NotesDateTime(dtEnd)

		GetBusinessDays = 0
		bd = Evaluate({@BusinessDays([}&_
		Cdat(StartDT.DateOnly)& {];[}&_
		Cdat(EndDT.DateOnly)& {];}&_
		Me.nondays &{;}&_
		Me.holidays &_
		{)})
		GetBusinessDays = Cint(bd(0))
	End Function

End Class

To test the code, put the code into the declaration section of a button. In your “Click” event type

Sub Click(Source As Button)
	Dim dtStart As String
	Dim dtEnd As String
	dtStart = "05.06.2007"
	dtEnd = "11.06.2007"

	'Dim b As New BusinessDay("1:7","[08.06.2007]:[06.06.2007]:[07.06.2007]")
	'Dim b As New BusinessDay("","[08.06.2007]:[06.06.2007]:[07.06.2007]")
	Dim b As New BusinessDay("","")
	Msgbox b.GetBusinessDays (dtStart,dtEnd )
End Sub