Having problems with the range!

Your range reference in red needs to refer to a worksheet first.

Sub WeekA()
Dim ws As Worksheet
Set ws = ActiveSheet

Dim Previousrow As Long
Previousrow = Cells(Rows.Count - 5, "B").End(xlUp).Row 'this isn't going to do what you think it is going to do

Dim Lastrow As Long
Lastrow = Cells(Rows.Count, "B").End(xlUp).Row

Dim Newrow As Long
'Newrow = Cells(Rows.Count + 5, "B").End(xlUp).Row 'This will error.  Rows.Count + 5 is like saying infinity plus 5.  Just doesn't work

'If Newrow worked; Previousrow, Lastrow, & Newrow will all give the same row (unless all rows in your worksheet are full).  You are doing something wrong
MsgBox (Previousrow & " " & Lastrow) 'example

Sheets("Sheet2").Range("A3:I7").Copy ws.Range("A" & Rows.Count).End(3)(2)
Range("B126:C130").AutoFill Destination:=Range("B126:C135"), Type:=xlFillWeekdays 'needs to reference a worksheet in both first range and destination range

'If you want to incorporate first and last row into a range it would be something like this; with a proper previousrow
'ws.Range("B" & Previousrow, "C" & Lastrow).AutoFill Destination:=ws.Range("B" & Previousrow, "C" & Lastrow), Type:=xlFillWeekdays

End Sub

I've managed to play around a bit and done it quite cheekily I think...
Here is the code for anybody interested...

Sub WeekA()
Dim ws As Worksheet
Set ws = ActiveSheet

Dim Previousrow As Long
Previousrow = Range("A" & Rows.Count).End(xlUp).Offset(1).Row

Sheets("Sheet2").Range("A3:I7").Copy ws.Range("A" & Rows.Count).End(3)(2)

Dim Lastrow As Long
Lastrow = Range("A" & Rows.Count).End(xlUp).Row

Dim Newrow As Long
Newrow = Range("A" & Rows.Count).End(xlUp).Offset(5).Row

ws.Range("B" & Previousrow, "C" & Lastrow).Offset(-5, 0).AutoFill Destination:=ws.Range("B" & Previousrow, "C" & Newrow).Offset(-5, 0), Type:=xlFillWeekdays

End Sub

Thanks for the help!!