Sub test()
    Dim myRng As Range, i As Long
    
    Set myRng = Cells(Rows.Count, 1).End(xlUp)
    For i = myRng.Row To 1 Step -1
        With Cells(i, 1)
            If .Value = "六" Then
                .Borders(xlEdgeBottom).LineStyle = xlDouble
                Exit For
            End If
        End With
    Next i
    
    i = i - 14
    Do Until i <= 0
      With Cells(i, 1)
          If .Value = "六" Then
              .Borders(xlEdgeBottom).LineStyle = xlDouble
          End If
      End With

      i = i - 14
    Loop
End Sub

應該是這樣吧
我不會寫vba 所以也不確定@@

For i = 1 To 60
    If Cells(i * 14, 1) = "六" Then
        Cells(i * 14, 1).Font.Underline = xlUnderlineStyleSingle
    End If
Next

看了一下你要從最後一列找
那應該是這樣

For i = lastSaturday To 1 Step -1
    If i Mod 14 = 0 And Cells(i, 1) = "六" Then
            Cells(i, 1).Font.Underline =xlUnderlineStyleSingle
    End If
Next

Private Sub Test()
    Dim lastRow As Integer
    lastRow = Cells(1, 1).End(xlDown).Row
    Dim i As Integer
    For i = lastRow To 1 Step -1
        If Cells(i, 1).Value = "六" Then
            Cells(i, 1).Font.Underline = xlUnderlineStyleDoubleAccounting
            If i - 14 > 0 Then
                Cells(i - 14, 1).Font.Underline = xlUnderlineStyleDoubleAccounting
            End If
            MsgBox ("完成!")
            Exit For
        End If
    Next
End Sub

底線樣式可以看 這裡