You don't need any interaction with the workbook to get the weekend date (ISO).
No benefit to put it into a separate function either.
Sub M_snb()
'saturday
MsgBox DateAdd("d", 6-Weekday(Date, 2), Date)
'sunday
MsgBox DateAdd("d", 7-Weekday(Date, 2), Date)
End Sub
E.g the Initialize Event in Userform1:
Private Sub UserForm_Initialize()
' UserForm5.Show
' Call Module1.todaysDate ''call date function to fill text box on form
' Call Module1.centerForm(UserForm1) ''call center function to center form in view
' todaysDate ''set text box to date
' Call Module1.sDate ''call sunday date func
' sDate ''set text box to sunday date
''POPULATES COMBO BOXES WITH COST CODES from table reference
' With Me
' .CostCode2.List = codes
' .CostCode2.List = codes
' .CostCode3.List = codes
' .CostCode4.List = codes
' .CostCode5.List = codes
' .CostCode6.List = codes
' End With
startupposition = 2 ' should be stored in design mode
dateIn.Value = Date
sundayDate.Value = DateAdd("d", 7 - Weekday(Date), Date)
sn = Range("cCodes")
For j = 1 To 5
Me("CostCode" & j).List = sn
Next
End Sub