before save event, need help for looping two cells at the same time

I'm trying to do a beforesave event, not allowing users to save if one of two given cells are empty. What I managed to do so far is linking column 13 (M) and cell A4.

What I'd like to do is applying the event to a combination of two range and rows, A4-A19 and M4-M19. In this way: If A4 is not empty and M4 is empty, a msgbox appears and blocks saving and so on..A5-M5, A6-M6...until A19-M19. If both corresponding cells are empty at the same time, then saving should be possible.

Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)

    Dim i As Integer, MyWb As Object
    i = 13
    Set MyWb = ThisWorkbook.Sheets("Planning").Cells
    Do While MyWb(4, i).Value <> ""
    i = i + 1
    Loop
    If i = 13 Then
        If ThisWorkbook.Sheets("Planning").Range("A4") <> "" Then
            MsgBox ("You will need to enter topics before saving"), vbCritical
            Cancel = True
        End If
    End If
End Sub

Based on Wolfie's code, I managed to obtain what I wanted, just adding a If not isempty for A column and replacing 19 instead of 13.

Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)

Dim plansht As Worksheet
Set plansht = ThisWorkbook.Sheets("Planning")

' Loop over rows
Dim rw As Integer
For rw = 4 To 19
    ' Test if both the A and M column in row "rw" are blank
    If Not IsEmpty(plansht.Range("A" & rw)) And plansht.Range("M" & rw).Value = "" Then
        MsgBox ("You will need to enter topics before saving"), vbCritical
        Cancel = True
    End If
Next rw

End Sub

Solution

You can loop over the rows, and just test the A and M columns to test if they are both blank for a given row. See the below code...

Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)

Dim plansht as Worksheet
Set plansht = ThisWorkbook.Sheets("Planning")

' Loop over rows
Dim rw as Integer
For rw = 4 to 13  
    ' Test if both the A and M column in row "rw" are blank
    If plansht.Range("A" & rw).Value = "" And plansht.Range("M" & rw).Value = "" Then
        MsgBox ("You will need to enter topics before saving"), vbCritical
        Cancel = True   
    End If    
Next rw

End Sub

Edit:

Your edit suggested you want some different combinations of the cells being empty. Here are some examples of the If statement for different outcomes

' If BOTH are empty
If plansht.Range("A" & rw).Value = "" And plansht.Range("M" & rw).Value = "" Then ...

If IsEmpty(plansht.Range("A" & rw)) And IsEmpty(plansht.Range("M" & rw)) Then ...

' If EITHER is empty
If plansht.Range("A" & rw).Value = "" OR plansht.Range("M" & rw).Value = "" Then ...

If IsEmpty(plansht.Range("A" & rw)) Or IsEmpty(plansht.Range("M" & rw)) Then ...

' If BOTH are NOT empty
If plansht.Range("A" & rw).Value <> "" And plansht.Range("M" & rw).Value <> "" Then ...

If Not IsEmpty(plansht.Range("A" & rw)) And Not IsEmpty(plansht.Range("M" & rw)) Then ...

Notice that when you start introducing Not with multiple conditions, the logic can quickly become hard to interpret. You can use brackets to group conditions with a Not, but you get things like this meaning the same logically:

If Not IsEmpty(plansht.Range("A" & rw)) And Not IsEmpty(plansht.Range("M" & rw)) Then ...
If Not (IsEmpty(plansht.Range("A" & rw)) Or IsEmpty(plansht.Range("M" & rw))) Then ...