Use variables, something along the lines of this.
Had to format the date to eliminate forward slashes which are illegal in the file name.
Code:
Sub Try_This()
Dim strFilename As String, D As Workbook
Dim newWeekNo As String, newWeekStart As String
Set D = ActiveWorkbook
newWeekNo = Range("sWeekNo") + 1
newWeekStart = Format(Range("sWeekStart") + 7, "d-mmm-yy")
strFilename = D.Path & "\GJCT Roster Week " & newWeekNo & " WS " & newWeekStart & ".xlsm"
D.SaveCopyAs Filename:=strFilename
Workbooks.Open (strFilename)
Range("sWeekNo") = newWeekNo
Range("sWeekStart") = newWeekStart
D.Close
End Sub